Minimal changes to get a clean compilation of the SAMD20 Xplained configuration. Still a lot of missing logic
Add SAMD20 pin configuration definitions
SAMD20: Add port configuration logic
SAMD20:  Add GCLK and PM register definition header files
SAMD20:  Add SYSCTRL register definition header file
SAMD20: Fixes to SRAM size, No. memory regions, and beginning of clocking logic
Add initial support for the waveshare open4357 demo board, only UART working

Signed-off-by: David G <david.guyueyi@gmail.com>