[WATCHDOG] set max63xx driver as ARM only

Use of ioremap() causes build failure on S390.
Restrict the driver to ARM until another architecture comes along
and enables the driver for its own use.

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>