soc: select CONFIG_CPU_CORTEX_M_HAS_VTOR for SAMD2x.

SAMD2x series have available the Optional Vector Table Offset Register
(VTOR). If this option is not selected, SCB->VTOR is not configured
properly and the application hangs if it's located in an offset
different than 0.

Removed select CPU_CORTEX_M from SAMD20, as it's already selected by
select CPU_CORTEX_M0PLUS

Signed-off-by: Miguel A. Borrego <jimjim17@gmail.com>