msc_sm2s_imx8mp: Adjust the initrd_addr location

Booting an initramfs with the current initrd_addr address may lead to
initramfs corruption and boot failure.

Fix the initramfs problem by applying the following layout
suggested by Tom Rini:

loadaddr=0x40480000 --> Gets moved to 0x40600000 in run-time:

   Uncompressing Kernel Image
Moving Image from 0x40480000 to 0x40600000, end=41e80000

fdt_addr_r= moved loadaddr + 128 MiB = 0x48600000

initrd_addr=fdt_addr_r + 512 KiB = 0x48680000

Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>