mfd: syscon: Set regmap max_register in of_syscon_register

Determine the regmap max_register configuration from the io resource size
and the reg-io-width device tree property.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>