pinmux: Expand the pin function/mode parameter size

The pinmux API was modified to expand the meaning of the 'func' argument
to allow it to represent more than a pre-configured function.  This was done
to reasonably accommodate a larger range of pin configuration options
offered by other MCUs, such as the Freescale K64 (up to 8 pin functions,
plus interrupt, pullup/down, drive strength, open-drain, slew rate, etc.).

This allows bit fields to be used to define various settings.

Change-Id: I2b216b822c6bae7133eed01c8c3339bb47b6c5db
Signed-off-by: Jeff Blais <jeff.blais@windriver.com>