riscv: Add FPIOA and GPIO support for Kendryte K210

This patch adds the necessary configs and docs for FPIOA and GPIO support
on the K210.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
diff --git a/board/sipeed/maix/Kconfig b/board/sipeed/maix/Kconfig
index 0cdcd32..4c42dd2 100644
--- a/board/sipeed/maix/Kconfig
+++ b/board/sipeed/maix/Kconfig
@@ -44,4 +44,13 @@
 	imply RESET_SYSCON
 	imply SYSRESET
 	imply SYSRESET_SYSCON
+	imply PINCTRL
+	imply PINCONF
+	imply PINCTRL_K210
+	imply DM_GPIO
+	imply DWAPB_GPIO
+	imply SIFIVE_GPIO
+	imply CMD_GPIO
+	imply LED
+	imply LED_GPIO
 endif