powerpc/watchdog: replace CONFIG_FSL_BOOKE with CONFIG_PPC_FSL_BOOK3E

CONFIG_FSL_BOOKE is only defined in 32-bit, CONFIG_PPC_FSL_BOOK3E is
defined in both 32-bit and 64-bit, so use CONFIG_PPC_FSL_BOOK3E to make
driver work in 32-bit & 64-bit.

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