include: configs: am**x/j721e/j721s2_evm.h: Move the stack pointer init address in arm64

Currently, in case of arm64 bootloader and U-Boot the stack pointer is
initialized at an offset of NON_SECURE_MSRAM_SIZE from arm64 SPL's text
base address. After jumping to arm64, execution is done out of DDR.
Therefore, having an offset corresponding to the size of MSRAM does not
have any significance.

Instead, initialize the stack pointer after an offset of 4MB from the SPL
text base address. This helps in allocating larger memory for stack.

  ┌────────────────────┐0x80080000
  │                    │
  │   arm64 SPL        │
  ├────────────────────┤
  │        ▲           │
  │        │           │
  │      STACK         │
  ├────────────────────┤0x80480000
  │ Memory for Load    │
  │ Buffer Allocation  │
  ├────────────────────┤0x80800000
  │                    │
  │    U-Boot Image    │
  │                    │
  └────────────────────┘

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>