pinctrl: Add RISC-V Canaan Kendryte K210 FPIOA driver

Add the pinctrl-k210.c pinctrl driver for the Canaan Kendryte K210
field programmable IO array (FPIOA) to allow configuring the SoC pin
functions. The K210 has 48 programmable pins which can take any of 256
possible functions.

This patch is inspired from the k210 pinctrl driver for the u-boot
project and contains many direct contributions from Sean Anderson.

The MAINTAINERS file is updated, adding the entry "CANAAN/KENDRYTE K210
SOC FPIOA DRIVER" with myself listed as maintainer for this driver.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>