gpiolib: of: make Freescale SPI quirk similar to all others

There is no need for of_find_spi_cs_gpio() to be different from other
quirks: the only variant of property actually used in DTS is "gpios"
(plural) so we can use of_get_named_gpiod_flags() instead of recursing
into of_find_gpio() again.

This will allow us consolidate quirk handling down the road.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>