arm: socfpga: misc: Fix warm reset

Write necessary magic value into the Warm Boot from ON-Chip RAM
group Enable register to enable Warm reset support. Instead of
doing this in the reset_cpu() function, we do it in arch early
init to avoid breaking old kernel code which expects this magic
value to be already written into this register.

This magic is originally excavated from common/spl/spl.c in the
u-boot port from altera, where this value was written just before
the SPL jumped to actual U-Boot in the RAM.

Signed-off-by: Marek Vasut <marex@denx.de>