ov5640_mipi: use registers 0x3602-3603 for step focus

Bits 4-7 of r3602 and bits 0-5 of r3603 comprise a 10-bit
field for VCM (voice coil motor) target field. Map these
to an 8-bit control to match the 8-bit field for the OV5642.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>