ft5x06: add support for reading/writing registers

Add sysfs entry 'reg' to read/write registers.

To read a register, write the register number to the sysfs
entry and then read:
	root@nitrogen6x:~# echo 0x80 > /sys/bus/i2c/drivers/ft5x06-ts/2-0038/reg
	root@nitrogen6x:~# cat /sys/bus/i2c/drivers/ft5x06-ts/2-0038/reg
	ft5x06[80]=14

To write a register, write "regnum=value" to sysfs:
	root@nitrogen6x:~# echo "0x80=0x01" > /sys/bus/i2c/drivers/ft5x06-ts/2-0038/reg

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