ARM: keystone2: spl: Fix stack allocation with CONFIG_SYS_MALLOC_F_LEN

If CONFIG_SYS_MALLOC_F_LEN is enabled, the stack is moved down to the
specified size to make the malloc function available before relocation.
But on keystone platforms SYS_SPL_MALLOC is immediately preceding stack,
which is causing an overlap with this config enabled.
So leave a gap between malloc space and stack space.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>