clk: at91: clk-system: add support for parent_hw

Add support for parent_hw in system clock drivers.
With this parent-child relation is described with pointers rather
than strings making registration a bit faster.

All the SoC based drivers that rely on clk-system were adapted
to the new API change. The switch itself for SoCs will be done
in subsequent patches.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20230615093227.576102-7-claudiu.beznea@microchip.com