ov5640_mipi: add support for reading/writing I2C registers

Read a register by setting the register
	/sys/bus/i2c/devices/5-003e# echo 3002 > ov5640_reg
	/sys/bus/i2c/devices/5-003e# cat ov5640_reg
	ov5640[0x3002]=0x1c

Write a register using regnum=value (in hex):
	ov5640[0x3002]=0x1c
	/sys/bus/i2c/devices/5-003e# echo 3603=1f > ov5640_reg
	/sys/bus/i2c/devices/5-003e# cat ov5640_reg
	ov5640[0x3603]=0x1f

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