booke/watchdog: refine and clean up the codes

Basically, this patch does the following:
1. Move the codes of parsing boot parameters from setup-common.c
   to driver. In this way, code reader can know directly that
   there are boot parameters that can change the timeout.
2. Make boot parameter 'booke_wdt_period' effective.
   currently, when driver is loaded, default timeout is always
   being used in stead of booke_wdt_period.
3. Wrap up the watchdog timeout in device struct and clean up
   unnecessary codes.

Signed-off-by: Tang Yuantian <yuantian.tang@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Li Yang <leoli@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>