Input: atmel_mxt_ts - add variable length __mxt_write_reg

The i2c bus requires 4 bytes to do a 1-byte write
(1 byte i2c address + 2 byte offset + 1 byte data).

By taking a length with writes, the driver can amortize transaction
overhead by performing larger transactions where appropriate.

This patch just sets up the new API.  Later patches refactor writes
to take advantage of the larger transactions.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>