arm: socfpga: execute cold reboot by default

This changes system reboot for socfpga to issue a cold reboot by
default instead of a warm reboot.

Warm reboot can still be used by setting reboot_mode to
REBOOT_WARM (e.g. via kernel command line 'reboot='), but this
patch ensures cold reboot is issued for both REBOOT_COLD and
REBOOT_HARD.

Also, cold reboot is more fail safe than warm reboot has some
issues at least fo CSEL=0 and BSEL=qspi, where the boot rom does
not set the qspi clock to a valid range.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>