imx: imx_clock: wdog: Add watchdog clock API

imx: imx_clock: wdog: Add watchdog clock API

This patch adds a set of functions to enable the clock for each of the
watchdog IP blocks.

Unlike the MMC and UART blocks, the watchdog blocks operate off of the one
root clock, only the clock-gates are enable/disabled individually.

As a consequence the function clock_set_wdog_clk_root_bits() is used to set
the root-slice just once for all of the watchdog blocks.

Future implementations may need to change this model but for now on the one
supported processor and similar NXP SoCs this model should work fine.

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