overlays: uart0 - return GPIOs 14 and 15 to inputs

In the event that alternate pins are used (only useful on Compute
Modules), return the standard pins to inputs to avoid double-mapping
them.

See: https://www.raspberrypi.org/forums/viewtopic.php?p=1388713#p1316977



Signed-off-by: Phil Elwell <phil@raspberrypi.org>