regulator: arizona-micsupp: Add missing #include

of.h is presently being included through asm-generic/gpio.h so will not
be included on some architectures, causing implicit declaration errors
for of_get_child_by_name, of_parse_phandle and of_node_put.

This patch adds the direct include that should be there.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>