[ARM] Save boot register values passed by bootloader

The ARM bootloader pass information to ARM Linux via r0, r1, and r2
registers.

The Xvisor boot protocol is same as that of ARM Linux hence we save
the values of r0, r1, and r2 so that in-future it is usefule for
getting DTB passed to Xvisor via bootloader.

Signed-off-by: Anup Patel <anup@brainfault.org>