armv8: mach-k3: correct define checking for AM625/AM642 memory maps

Using CONFIG_IS_ENABLED breaks accessing memory map structure when
doing a A53 SPL build for AM625 and AM642 platforms. This is due to
'abc if CONFIG_SPL_BUILD is defined and CONFIG_SPL_FOO is set to 'y''
in which there is no CONFIG_SPL_SOC_K3_AM625/CONFIG_SPL_SOC_K3_AM642
defined in the configuration.

For the A53 SPL builds on these platform to access the memory mapping
which it will need for enabling the mmu/cache it must use #if defined(X)
checks and not CONFIG_IS_ENABLED.

Cc: Suman Anna <s-anna@ti.com>
Cc: Neha Francis <n-francis@ti.com>
Signed-off-by: Matt Ranostay <mranostay@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>