ARM: tegra: turn on UART A clock at boot

ARM: tegra: turn on UART A clock at boot

Some boards use UART D for the main serial console, and some use UART A.
UART D's clock is listed in board-dt-tegra20.c's clock table, whereas
UART A's clock is not. This causes the clock code to think UART A's
clock is unsed. The common clock framework turns off unused clocks at
boot time. This makes the kernel appear to hang. Add UART A's clock into
the clock table to prevent this. Eventually, this requirement should be
handled by the UART driver, and/or properties in a board-specific device
tree file.

Signed-off-by: Stephen Warren <swarren@nvidia.com>