gpio: mxc_gpio: fix reading state of GPIO pins in output mode

The PSR register works correctly for GPIO pins in input mode,
but always returns 0 for GPIO pins in output mode unless the SION
bit is set.

The DR register should be used for GPIO pins in output mode
to allow correct getting of previously set output value.

Please note that the Linux gpio-mxc driver and the NXP U-Boot mxc_gpio
driver already use the DR register for all GPIO pins in output mode:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=442b2494b17d1a4f0a14721580271eb23ebffd42
https://github.com/nxp-imx/uboot-imx/commit/4afc3f90943c6b117f79b66d2cd04e64f437b0c2



Signed-off-by: Tomas Paukrt <tomaspaukrt@email.cz>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Fabio Estevam <festevam@gmail.com>