mips: gpio: mscc: Obsoleted gpio-mscc-bitbang-spi.c

With the new mscc_bb_spi.c driver, there is no longer use for the
gpio-mscc-bitbang-spi.c driver.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>