ov5642_camera: add sysfs entry to read/write I2C

Use /sys/bus/i2c/devices/i2c-1/1-003d/ov5642_reg to read
a register like so:
	root@nitrogen6x:~# echo 0x4303 > /sys/bus/i2c/devices/i2c-1/1-003d/ov5642_reg
        root@nitrogen6x:~# cat /sys/bus/i2c/devices/i2c-1/1-003d/ov5642_reg
	0xff

Or write a register with a second parameter:
	root@nitrogen6x:~# echo 0x4303 0xac > /sys/bus/i2c/devices/i2c-1/1-003d/ov5642_reg
        root@nitrogen6x:~# cat /sys/bus/i2c/devices/i2c-1/1-003d/ov5642_reg
	0xac

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