gpio/mpc83xx_spisel_boot.c: gpio driver for SPISEL_BOOT signal

Some SoCs in the mpc83xx family, e.g. mpc8309, have a dedicated spi
chip select, SPISEL_BOOT, that is used by the boot code to boot from
flash.

This chip select will typically be used to select a SPI boot
flash. The SPISEL_BOOT signal is controlled by a single bit in the
SPI_CS register.

Implement a gpio driver for the spi chip select register. This allows a
spi driver capable of using gpios as chip select, to bind a chip select
to SPISEL_BOOT.

It may be a little odd to do this as a GPIO driver, since the signal
is neither GP or I, but it is quite convenient to present it to the
spi driver that way. The alternative it to teach mpc8xxx_spi to handle
the SPISEL_BOOT signal itself (that is how it's done in the linux
kernel, see commit 69b921acae8a)

Signed-off-by: Klaus H. Sorensen <khso@prevas.dk>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index f751a8b..2081520 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -423,6 +423,14 @@
 	  value setting, the open-drain feature, which can configure individual
 	  GPIOs to work as open-drain outputs, is supported.
 
+config MPC83XX_SPISEL_BOOT
+	bool "Freescale MPC83XX SPISEL_BOOT driver"
+	depends on DM_GPIO && ARCH_MPC830X
+	help
+	  GPIO driver to set/clear dedicated SPISEL_BOOT output on MPC83XX.
+
+	  This pin is typically used as spi chip select to a spi nor flash.
+
 config MT7621_GPIO
 	bool "MediaTek MT7621 GPIO driver"
 	depends on DM_GPIO && SOC_MT7628