flash: at91samd: add SAML21 support, fix part ID issue

flash: at91samd: add SAML21 support, fix part ID issue

This adds support for the new Atmel SAML21 family of low-power Cortex
M0+ devices.  Their Flash controller is essentially the SAMDxx one so
the change consists of adding the new part IDs.  Unfortunately the
device ID logic had a couple of mistakes in it that did not affect
anything on SAMD2x devices (due to 0 values expected there) but that is
a problem on L21, it's therefore addressed here and things should now
match the datasheets.

Tested on Amtel SAML21 Xplained Pro development kit against the included
SAML21J18A there.  Also tested for regressions on a SAMD20 and SAMD21
using their dev kits.

Change-Id: I768f75e064b8656c15148730dacaa4c3acfc4101
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/2690
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>