tegra: rename FUNCMUX_UART2_UARTB

FUNCMUX_ defines should be named after the pin groups they affect, not
after the module they're muxing onto those pin groups.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>