pinctrl: Rework the pinmux handling for BM1880 SoC

Rework the BM1880 SoC pinmux handling by removing the
BM1880_PINMUX_FUNCTION_MUX define and merging it with the
BM1880_PINMUX_FUNCTION definition. Since the PWM muxing is handled by
generic pin controller in the SoC itself, there is no need to have a
dedicated code to do the muxing in PWM registers. So, lets club all
pinmux handling in the same per pin mux handling code.

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