media: qcom: camss: csiphy-3ph: Use an offset variable to find common control regs

New versions of the CSIPHY locate the control registers at offset 0x1000
not offset 0x800.

Provide a variable to base an offset from for the purposes of redirecting
the base offset for the new PHY regs layout.

The existing setup bases from 0x800, the new from 0x1000 with some of the
'EXT' registers dropped but the lower-order lane config regs at offset 0x00
and up the same as before.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Depeng Shao <quic_depengs@quicinc.com>
Reviewed-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>