[WATCHDOG] fixed book E watchdog period register mask.

A previous fix changed the WDTP function to use the period directly,
rather than subtracting from 63.  However the mask generation was
not changed, so the mask was coming out as 0.  This patch fixes it.
Signed-off-by: NLuuk Paulussen <luuk.paulussen@alliedtelesis.co.nz>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>

drivers/watchdog/booke_wdt.c

@@ -44,7 +44,7 @@ u32 booke_wdt_period = WDT_PERIOD_DEFAULT;
 
 #ifdef	CONFIG_FSL_BOOKE
 #define WDTP(x)		((((x)&0x3)<<30)|(((x)&0x3c)<<15))
-#define WDTP_MASK	(WDTP(0))
+#define WDTP_MASK	(WDTP(0x3f))
 #else
 #define WDTP(x)		(TCR_WP(x))
 #define WDTP_MASK	(TCR_WP_MASK)