SAML21-Xplained:  Update the cloned SAMD20-Xplained board configuration for specifics of the SAML21-Xplained

SAMD/L: Hide differences in the way peripheral clocking is enabled behind macros.  Some updates to saml_clockconfig.h + various fixes while working toward SAML21 build

Clock configuration logic for the SAML21.  Initial commit is just the SAMD20 clock configuration with some naming changes and does not compile

Add basic board build configurat for the SAML21 Xplained.  Initial commit is jsut the SAMD20 Xplained with name changes and does not yet build

Update the type passed to watchdog timer handlers.  Using uint32_t is a problem for 64-bit machines.

SAMD/L: Move content of chip/sam_memorymap.h to chip.h; move chip/sam_pinmap. to sam_pinmap.h for consistency with other header files