drivers: sdhc: enable pwr-gpios property within SPI SDHC driver

Enable SPI SDHC driver to manage card power via pwr-gpios property.
Control for this property was previously partially implemented. When
this property is present, the SPI SDHC driver will use it to control
power to the SD card.

Power is toggled during SD init, so this power control can make SD init
more reliable as the power toggle will insure the SD card state is reset.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>