ARM: restart: allow platforms more flexibility specifying restart mode

Change 'soft_reboot' into a more generic 'restart_mode' variable,
allowing the default restart mode to be specified.

Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>