dt-bindings: serial: snps-dw-apb-uart: Allow use of a power-domain

dt-bindings: serial: snps-dw-apb-uart: Allow use of a power-domain

The UART controllers in most Rockchip SoCs are part of power domains
that are always powered on. These always powered on power domains have
typically not been described in the device tree.

Because these power domains have been left out of the device tree there
has not been any real need to properly describe the UART controllers
power domain of Rockchip SoCs.

On Rockchip RK3528 the UART controllers are spread out among the
described PD_RKVENC, PD_VO and PD_VPU power domains. However, one UART
controller belong to an undescribed always powered on power domain.

Add support to describe an optional power-domains for the UART
controllers.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250723085654.2273324-5-jonas@kwiboo.se
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>