spl: Kconfig: ARCH_K3: Set default SPL_STACK_R_MALLOC_SIMPLE_LEN for R5 build

spl: Kconfig: ARCH_K3: Set default SPL_STACK_R_MALLOC_SIMPLE_LEN for R5 build

All ARCH_K3 platforms need about of 2MB of malloc space post
reallocation. Since, this space is allocated from SDRAM, provide a
generous 2MB space by default.

Platforms requiring more than 2MB can override in defconfig as needed.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>