flash: sam0: Define LOCK_REGIONS in dts

The amount of lock regions differs between different sam0 MCUs.

	saml10:  2
	saml11:  4
	samd2x:	16
	saml2x: 16
	samd5x: 32

ASF does not provide a definition for this, so create a new one
in dts.

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>