imx7: imx7_clock: uart: Add UART clock init logic

imx7: imx7_clock: uart: Add UART clock init logic

This patch adds an internal UART init routine that gets called from the
external facing clock init function.

In the first pass this call does an explicit disable of all UART
clock-gates. Later changes will enable only the UART clock-gates we care
about.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>