Add APB DMA requestor and serial aliases for serial controller.
There will be two serial driver i.e. 8250 based simple serial driver
and APB DMA based serial driver for higher baudrate and performace.

The simple serial driver get enabled with compatible nvidia,tegra20-uart
and APB DMA based driver will get enabled with compatible
nvidia,tegra20-hsuart.
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>