flash: at91samd: Add SAML21 variant B device support and fix SAMC20/SAMC21

This adds support for the Atmel SAML21 variant B parts.
There is minimal change between the two variants, but in
variant B the automatic page write which the at91samd flash
driver relies on to be enabled is disabled by default.
With this patch the write row function will now issue a page write
command after each of the four pages in the row if the MANW (manual
write) bit is set. This also fixes flash write for the SAMC20/SAMC21
devices which have the MANW bit set by default as well.

I have also moved the device ID (DID) register bitfield extraction
from the find_part into helper macros. These can be used in the future
if there are more workarounds for specific devices.

Tested (programming) on:
ATSAML21-XPRO
ATSAML21-XPRO-B
SAMC21 Xplained Pro
SAMD21 Xplained Pro
SAMD20 Xplained Pro

Change-Id: I401a8aa1efd64730840c0d62cf49a1e880ea5900
Signed-off-by: Andreas Loehre <alohre@gmail.com>
Reviewed-on: http://openocd.zylin.com/2903


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>