powerpc/watchdog: Make default timeout for Book-E watchdog a Kconfig option

The PowerPC Book-E watchdog driver (booke_wdt.c) defines a default timeout
value in the code based on whether it's a Freescale Book-E part of not.
Instead of having hard-coded values in the driver, make it a Kconfig
option.

As newer chips gets faster, the current default values become less
appropriate, since the timeout sometimes occurs before the kernel finishes
booting.  Making the value a Kconfig option allows BSPs to configure a new
value without requiring the wdt_period command-line parameter to be set.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>