watchdog: intel_scu_watchdog: fix Kconfig dependency

Kernel symbol X86_MRST has been removed from the kernel.
INTEL_SCU_WATCHDOG driver can never be compiled due dependence of X86_MRST
which remained in the drivers/watchdog/Kconfig.

Reported-by: Alexander Shiyan <shc_work@mail.ru>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>