phy: rockchip: phy-rockchip-inno-csidphy: allow writes to grf register 0

phy: rockchip: phy-rockchip-inno-csidphy: allow writes to grf register 0

The driver for the Rockchip MIPI CSI-2 DPHY uses GRF register offset
value 0 to sort out undefined registers. However, the RK3588 CSIDPHY GRF
this offset is perfectly fine (in fact, register 0 is the only one in
this register file).
Introduce a boolean variable to indicate valid registers and allow writes
to register 0.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Michael Riesch <michael.riesch@collabora.com>
Link: https://lore.kernel.org/r/20250616-rk3588-csi-dphy-v4-4-a4f340a7f0cf@collabora.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>