armv8: fsl-layerscale: Rewrite reserving memory for MC and debug server

MC and debug server are not board-specific. Move reserving memory to SoC
file, using the new board_reserve_ram_top function. Reduce debug server
memory by 2MB to make room for secure memory.

In the system with MC and debug server, the top of u-boot memory
is not the end of memory. PRAM is not used for this reservation.

Signed-off-by: York Sun <yorksun@freescale.com>