set pin modes for UART

[pdp7]

Use set_pin_mode() to ensure pinmux is correct for UART peripheral before using.

NOTE: GPIO and PWM already call set_pin_mode()

[pdp7]

My work-in-progress (WIP) is in branch config-pin. Most recent commit: b926084

[pdp7]

For reference, config-pin reads and writes to files in /sys/devices/platform/ocp such as this path for P9_26:

debian@beaglebone:~$ cat /sys/devices/platform/ocp/ocp:P9_26_pinmux/state
uart
debian@beaglebone:~$ echo gpio > /sys/devices/platform/ocp/ocp:P9_26_pinmux/state
debian@beaglebone:~$ cat /sys/devices/platform/ocp/ocp:P9_26_pinmux/state
gpio
debian@beaglebone:~$ echo uart > /sys/devices/platform/ocp/ocp:P9_26_pinmux/state
debian@beaglebone:~$ cat /sys/devices/platform/ocp/ocp:P9_26_pinmux/state
uart