cnss: Implement API to store WLAN MAC address in platform driver

WLAN Functional Drivers Queries cnss platform driver to get the
MAC Address. If the OEM doesn't provide the valid MAC address, the
WLAN Driver fallbacks to use other approaches to get MAC address.

This works under CONFIG_CNSS_MAC feature flag, which will be enabled
only on the OEM platforms. For internal platforms, CNSS driver doesn't
hold any valid mac addresses.

CRs-Fixed: 985585
Change-Id: I1e8a030a32a640cec84cadd6b36b37938d5fe6be
Signed-off-by: Komal Kumar <kseelam@codeaurora.org>