powerpc/watchdog: include linux/processor.h for spin_until_cond

This implementation uses spin_until_cond in wd_smp_lock including
neither linux/processor.h nor asm/processor.h

This patch includes linux/processor.h here for spin_until_cond usage.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5e8d2d50f301a346040362028c2ecba40685de9e.1623438544.git.christophe.leroy@csgroup.eu