watchdog: ulp_wdog: Update watchdog driver for imx93

The WDOG clocks are sourced from the fixed 32KHz (lpo_clk).When the
timeout period exceeds 2 seconds, the value written to the TOVAL
register is larger than 16-bit can represent. Enabling watchdog
prescaler to solve this problem.

Signed-off-by: Alice Guo <alice.guo@nxp.com>
Reviewed-by: Stefan Roese <sr@denx.de>