watchdog: rzv2h: Make "oscclk" and reset controller optional

Update the rzv2h_wdt driver to make the "oscclk" clock and reset
controller optional.

Use devm_clk_get_optional_prepared() to obtain the "oscclk" clock,
allowing the driver to work on platforms that do not provide this clock,
such as the RZ/T2H SoC.

Similarly, use devm_reset_control_get_optional_exclusive() to allow the
driver to function on platforms that lack a reset controller.

These changes are preparatory steps for supporting the RZ/T2H SoC, which
does not provide an "oscclk" clock or a reset controller.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>