[efr32] fix COAPS switch build error (#4310)
[efr32] fix COAPS switch build error (#4310)

third_party/mbedtls/ecp.c was excluded for mg12 and mg21.
jwhui/openthread,chshu/openthread,librasungirl/openthread,lanyuwen/openthread,abtink/openthread,bukepo/openthread,srickardti/openthread,bukepo/openthread,abtink/openthread,librasungirl/openthread,srickardti/openthread,librasungirl/openthread,abtink/openthread,chshu/openthread,librasungirl/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,bukepo/openthread,lanyuwen/openthread,lanyuwen/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread,openthread/openthread,lanyuwen/openthread,chshu/openthread,bukepo/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread,lanyuwen/openthread,openthread/openthread,jwhui/openthread