Otherwise pinctrl won't work. Because of silicon errata for some dm814x
versions, let's also keep bit 18 out of the function-mask and rely on
the bootloader configuration for bit 18  as suggested by
Matthijs van Duin <matthijsvanduin@gmail.com>.

Devices with that need to use bit 18 can override the function-mask in
the board specific dts file if really needed.

Signed-off-by: Tony Lindgren <tony@atomide.com>