Kconfig: j721e: Change K3_MCU_SCRATCHPAD_BASE to non firewalled region

In non-combined boot flow for K3, all the firewalls are locked by default
until sysfw comes up. Rom configures some of the firewall for its usage
along with the SRAM for R5 but the PSRAM region is still locked.

The K3 MCU Scratchpad for j721e was set to a PSRAM region triggering the
firewall exception before sysfw came up. The exception started happening
after adding multi dtb support that accesses the scratchpad for reading
EEPROM contents.

The commit changes R5 MCU scratchpad for j721e to an SRAM region.

Old Map:
┌─────────────────────────────────────┐ 0x41c00000
│                 SPL                 │
├─────────────────────────────────────┤ 0x41c40000 (approx)
│                STACK                │
├─────────────────────────────────────┤ 0x41c85b20
│             Global data             │
│  sizeof(struct global_data) = 0xd8  │
├─────────────────────────────────────┤ gd->malloc_base = 0x41c85bfc
│                HEAP                 │
│  CONFIG_SYS_MALLOC_F_LEN = 0x70000  │
├─────────────────────────────────────┤ CONFIG_SPL_BSS_START_ADDR
│               SPL BSS               │ (0x41cf5bfc)
│  CONFIG_SPL_BSS_MAX_SIZE = 0xA000   │
└─────────────────────────────────────┘ CONFIG_SYS_K3_BOOT_PARAM_TABLE_INDEX
			                (0x41cffbfc)

New Map:
┌─────────────────────────────────────┐ 0x41c00000
│                 SPL                 │
├─────────────────────────────────────┤ 0x41c40000 (approx)
│                EMPTY                │
├─────────────────────────────────────┤ 0x41c81920
│                STACK                │
│ SPL_SIZE_LIMIT_PROVIDE_STACK=0x4000 │
├─────────────────────────────────────┤ 0x41c85920
│             Global data             │
│  sizeof(struct global_data) = 0xd8  │
├─────────────────────────────────────┤ gd->malloc_base = 0x41c859f0
│                HEAP                 │
│  CONFIG_SYS_MALLOC_F_LEN = 0x70000  │
├─────────────────────────────────────┤ CONFIG_SPL_BSS_START_ADDR
│               SPL BSS               │ (0x41cf59f0)
│  CONFIG_SPL_BSS_MAX_SIZE = 0xA000   │
├─────────────────────────────────────┤ 0x41cff9fc
│         NEW MCU SCRATCHPAD          │
│  SYS_K3_MCU_SCRATCHPAD_SIZE = 0x200 │
└─────────────────────────────────────┘ CONFIG_SYS_K3_BOOT_PARAM_TABLE_INDEX
			                (0x41cffbfc)

Fixes: ab977c8b ("configs: j721s2_evm_r5: Enable support for building multiple dtbs into FIT")

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
[n-francis@ti.com: SRAM allocation addressing diagram]
Signed-off-by: Neha Francis <n-francis@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Kamlesh Gurudasani <kamlesh@ti.com>