clk: tegra: initialise parent of uart clocks

Initialise the parent of UARTs to PLLP and disabling clock by
default.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>