OMAP2+: UART: Add mechanism to probe uart pins and configure rx wakeup

OMAP2+: UART: Add mechanism to probe uart pins and configure rx wakeup

The commit (bce492c0  ARM: OMAP2+: UART: Fix incorrect population of
default uart pads) removed default uart pads that where getting populated
and which was making rx pin wakeup capable. If uart pads were used in
different mode by any other module then it would fail since the default
pads took over all the uart pins forcefully. With removal of default pads
the rx_pad wakeup for console uart while waking up from off mode is broken.

Utilise the mux api available to probe the availability of mux pins
in uart mode and probe for availability of uart pin in mux mode0
if uart is available as uart pin itself then configure rx pin
as wakeup capable.

This patch itself doesn't cater to all boards. Boards using uart rx wakeup
mechanism should ensure the usage of omap_serial_init_port by configuring
required uart ports and pass necessary mux data, till then this probing of
uart pins can cater to enabling of rx pad wakeup to most of the boards.

This patch can also throw some boot warning from _omap_mux_get_by_name
if pin is requested for availability is not present while dynamically probing
the uart pins availability such boot warnings can be addressed only when board
files are patched with omap_serial_init_port calls passing the right pads
needed for a given port.

Discussion Threads for reference:
http://www.spinics.net/lists/linux-omap/msg69859.html
http://www.spinics.net/lists/linux-omap/msg68659.html

Cc: Felipe Balbi <balbi@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Russ Dill <russ.dill@gmail.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Ameya Palande <ameya.palande@ti.com>
Signed-off-by: Govindraj.R <govindraj.raja@ti.com>
[tony@atomide.com: updated to fix compile when CONFIG_OMAP_MUX is not set]
Signed-off-by: Tony Lindgren <tony@atomide.com>