eeprom_93cx6: Add data direction control.

Some devices need to know if the data is to be output or read, so add a
data direction into the eeprom structure to tell the driver whether the
data line should be driven.

The user in this case is the Micrel KS8851 which has a direction
control for the EEPROM data line and thus needs to know whether
to drive it (writing) or to tristate it for receiving.

Change-Id: Ic8bd7e13ce9bde423001ca23f3de02e76e1dbdaf
Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>