flash: Fix flash write algorithm on pflash only devices.

flash: Fix flash write algorithm on pflash only devices.

This patch fix the slow flash write issue on pflash only devices.
The Family Reference Manual says:

For devices with FlexNVM: FlexRAM
For devices with program flash only: Programming acceleration RAM

So the acceleration RAM is available for the flash section command on
this device.

Change-Id: If6541a23a4457c5ed8858848a145f35cac63138b
Signed-off-by: Mathias K <kesmtp@freenet.de>
Reviewed-on: http://openocd.zylin.com/509
Tested-by: jenkins
Reviewed-by: Tomas Frydrych <tf+openocd@r-finger.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>