pinctrl: Rework Kconfig dependency for BM1880 pinctrl driver

Make the BM1880 SoC pinctrl driver depend on COMPILE_TEST to get compiler
test coverage and also select this driver by default for ARCH_BITMAIN
platform.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>