atmel_lcdfb: Adjust HFP calculation so it matches the manual.

In the AT91SAM9263 Manual the HFP part in LCDTIM2 is described as follows:
  * HFP: Horizontal Front Porch
  Number of idle LCDDOTCK cycles at the end of the line.
  Idle period is (HFP+2) LCDDOTCK cycles.

It is only a minor issue. I also changed all boards using atmel_lcdfb
I found to respect the new calculation.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>