pinctrl: Add support for Kendryte K210 FPIOA

The Fully-Programmable Input/Output Array (FPIOA) device controls pin
multiplexing on the K210. The FPIOA can remap any supported function to any
multifunctional IO pin. It can also perform basic GPIO functions, such as
reading the current value of a pin. However, GPIO functionality remains
largely unimplemented (in favor of the dedicated GPIO peripherals).

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>