flash/nor/at91samd: add small delay before checking nvm status

OpenOCD's SWD subsystem doesn't currently have a consistent WAIT
handling (i.e. it doesn't ever retry, just returns an error), so right
after a row write a small delay is needed as AHB access is stalled
during the flashing operation.

The issue was exposed with a samd20 using ftdi SWD transport.

Change-Id: I07d99d3a96845cc689c3904a41f4d41344f200aa
Signed-off-by: Andrej Kazmin <funnyfish@funnyfish.botik.ru>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2268


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>