omap1: omap7xx clocks, mux, serial fixes

This change adds in the necessary clocks and mux pins for UART
control on omap7xx devices.  I also made a change in the serial
code to only try and initialize two UARTs in omap_serial_init, as
these devices don't have three.

Signed-off-by: Cory Maccarrone <darkstar6262@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>