Make the syscon-reboot driver accept value and mask instead of
just value.

Prior to this patch, the property name for the value was 'mask'. If
only the mask property is defined on a node, maintain compatibility
by using it as the value.

This patch is based on commit
f2c199db ("power: reset: syscon-poweroff: add a mask property")
and does the same change for the syscon-reboot driver.
Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>