gpio: moxart: get value of output gpio from generic driver

Adding a BGPIOF_READ_OUTPUT_REG_SET initialization flag to GPIO
generic MMIO driver makes possible to remove a private get() value
function from the driver.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>