Modify the UART initialization to allow the board-initialization code
to specify which pins are connected, and which pins should therefore
be initialized.

The current at91_init_serial() will continue to work as-is, but is
marked as "deprecated" and will be removed once the board-specific
files has been updated to use the new interface.

As in the AVR32 code, we assume that the TX and RX pins will always be
initialized.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>