configs: am62*_r5_usbmsc_defconfig: Set SPL_STACK_R_MALLOC_SIMPLE_LEN to 2M

Size of malloc was standardized to 2M for all am6*_r5_*_defconfigs, but
USB MSC defconfig was missed, update this config option now.

Fixes: 79461431

 (configs: am6*_r5_defconfig: Standardize SPL_STACK_R_MALLOC_SIMPLE_LEN to 2M)

Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>