gpio: mxc: read output value from GPIO_DR register

All supported iMX GPIO controllers store configured GPIO output value
in GPIO_DR data register, which is represented by GPIO generic reg_set.
Provide a BGPIOF_READ_OUTPUT_REG_SET flag to bgpio_init() to allow
correct getting of previously set output value.

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