[ARM] pxa: allow platforms to control which uarts are registered

For some platforms, it is inappropriate to register all PXA UARTs.
In some cases, the UARTs may not be used, and in others we may want
to avoid registering the UARTs to allow other drivers (eg, FICP) to
make use of the UART.

In addition, a while back there was a request to be able to pass
platform data to the UART driver.

This patch enables all of this by providing functions platforms can
call to register each individual UART.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>