GPIO: LPC32xx: Add output reading to GPO P3

The chip offers the function to detect the current state of output of the GPO
P3 pins. Useful for reading GPIO output state in Linux' GPIO API, e.g. via
sysfs.

Please note that this only reads back the currently programmed output state,
not the actual electrical level in terms of a GPI function. Finally, GPO3 is
still just an output.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>