pinmux: Add support for "runtime" pinmux on ARM V2M Beetle

This patch adds the support for "runtime" pinmux on ARM V2M Beetle.

The GPIO controllers 2 and 3 are reserved and therefore not exposed
by this driver.

Jira: ZEP-1245
Change-Id: I9637f1a0d2bf6a757e1942160fb170165ffe6a0c
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>