iio: magnetometer: mmc35240: Optionally get and handle reset

The MMC magnetometers appear to have an optional reset
line that is sometimes controlled by a GPIO. If this
is defined for the device, obtain it and de-assert
it right after obtaining the regulators. Assert the
reset line again on remove().

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>