cpu/sam0_common: update README for SAML1x addition

Rewrite the README to reflect the recent addition of SAMR30 and
SAML10/SAML11.

cpu/sam0_common/include/vendor/README.md

 # CMSIS from Atmel Software Foundation (ASF)
 
-The include files in the directory tree are copied from ASF (verion 3.35.1).
+The include files in this directory tree are copied from Atmel
+sources. Most of the sam0 files are from ASF (Atmel Software Foundation,
+version 3.35.1). The SAMR30 files are from ASF 3.34.2. The SAML10 and
+SAML11 files are from, so called, atpacks.
 
-The directory tree was copied "as is" from ASF path `sam0/utils/cmsis/` and its
-structure is as follows:
+## The sam0 files
+
+The directory tree was copied "as is" from ASF path `sam0/utils/cmsis/` and
+its  structure is as follows:
 
     cmsis
     ├── samd21
@@ -32,12 +37,29 @@ all `source` folders are removed.
 Be aware that if you want to make changes to any file in this tree that the
 changes will be lost when a new ASF release is going to be used.
 
-# Usage and porting for SAM based CPUs
+### sam0.h
+
+A SAM based CPU should include `sam0.h` in this directory, which will
+resolve  any CPU family specific includes required.
+
+## SAML10/SAML11 files
+
+Since 2019 the necessary variant files are available in atpacks at
+
+    http://packs.download.atmel.com/
+
+saml10: `Atmel.SAML10_DFP.1.0.142.atpack`
+saml11: `Atmel.SAML11_DFP.1.0.91.atpack`
+
+Each atpack has an include subdirectory with the files we copy into
+RIOT. The files are copied unmodified.
+
+### sam23.h
+
+A SAML1x based CPU should include `sam23.h` in this directory, which will
+resolve any CPU family specific includes required.
 
-SAM based CPU should include `sam0.h` in this directory, which will resolve any
-CPU family specific includes required.
+## Usage and porting for SAM based CPUs
 
-Currently only SAMD21, SAML21, and SAMR21 based CPUs are supported, i.e., only
-their CMSIS header files are copied from the ASF. If other CPUs are needed copy
-CMSIS files from the ASF for the respective CPU family here and adapt `sam0.h`
-accordingly.
+If other CPUs are needed, copy the include files from ASF or atpack for the
+respective  CPU family here and adapt `sam0.h` or `sam23.h` accordingly.