Add CONFIG_CLOCK_DFLL_RUNSTDY for SAMD21

[mws-rmain]

CONFIG_CLOCK_DFLL_RUNSTDY must exist & have a value or Harmony3 code generation will fail with: ===> CONFIG_CLOCK_DFLL_RUNSTDY [in template "plib_clock.c.ftl at line 195, column 43] has evaluated to null or missing.

These missing lines were copied from the existing SAML21 clk.py