riscv: rv32m1: Rework device_get_binding for pinmux

Switch to use DEVICE_DT_GET instead of device_get_binding for pinmux
device.  As part of this change drop the "label" property from
the pinmux devicetree node and update the binding and dts files to
reflect that.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>