ppc4xx: Sequoia has two UARTs in "4-pin" mode. Configure the GPIOs as per schematic.

The Sequoia board has two UARTs in "4-pin" mode. This patch modifies the GPIO
configuration to match the schematic, and also sets the SDR0_PFC1 register to
select the corresponding mode for the UARTs.

Signed-off-by: Steven A. Falco <sfalco@harris.com>
Signed-off-by: Stefan Roese <sr@denx.de>