gpio/mpc83xx_spisel_boot.c: gpio driver for SPISEL_BOOT signal

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>