pinctrl: add a Cirrus ep93xx SoC pin controller

Add a pin control (only multiplexing) driver for ep93xx SoC so
we can fully convert ep93xx to device tree.

This driver is capable of muxing ep9301/ep9302/ep9307/ep9312/ep9315
variants, this is chosen based on "compatible" in device tree.

Co-developed-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Tested-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>