gpio: mscc-bitbang-spi: Add a simple gpio driver for bitbgang spi

The VCore III SoCs such as the Luton but also the Ocelot can remap an SPI
flash directly in memory. However, for writing in the flash the
communication has to be done by software.

Each of the signal used for the SPI are exposed in a single register. In
order to be able to use the soft-spi driver, the management of this pin
is done through this simple gpio driver.

Even if the main purpose of this driver is to be used by soft-spi, it can
still be used as a normal gpio driver but with limitation: for example
the first pin can't be used as output.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>