powerpc/watchdog: move booke watchdog param related code to setup-common.c

Currently, BOOKE watchdog code for checking "wdt" and "wdt_period" is
in setup_32.c, it cannot be used in 64-bit, so move it to a common place
setup-common.c, which will be shared by 32-bit and 64-bit.

Also, replace the simple_strtoul with kstrtol.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>