boards: acrn: acrn: Updated soc name for acrn_ehl_crb

Updated soc name for acrn_ehl_crb to atom as
CONFIG_ATOM is selected for ACRN_EHL_CRB in
Konfig.acrn_ehl_crb but given soc elkhart_lake
in board.yml

Added common config ACRN_COMMON to enable common
configs of ACRN.

Signed-off-by: Anisetti Avinash Krishna <anisetti.avinash.krishna@intel.com>