Documentation: Document pm660(l) SPMI GPIOs compatible

Signed-off-by: Konrad Dybcio <konradybcio@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20200622192558.152828-3-konradybcio@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>