imx: Fix potential lmb memory overwritten by stack

At default, u-boot reserves the memory from SP - 4KB to DRAM end for
lmb in arch_lmb_reserve. So lmb won't allocate any memory from it.
But we found the 4K gap for SP is not enough now, because some FDT
updating operations are added in our u-boot before jumping to kernel,
which needs larger stack. This causes the lmb allocated memory is overwritten
by stack.

Fix the issue by implementing the board_lmb_reserve to reserve from
SP - 16KB to memory end for lmb.

Signed-off-by: Ye Li <ye.li@nxp.com>