cnss: Add support for DDR frequency voting

This is a power feature. To achieve better performace,
cnss driver exports a DDR frequency voting function
to wlan host driver. Wlan driver will call this
function to vote for different DDR frequency based on
its traffic bandwidth.

Change-Id: Iab932ea5acb43413cc18aa54e847f856648dac4b
CRs-Fixed: 627475
Acked-by: Sameer Thalappil <sameert@codeaurora.org>
Signed-off-by: Yuanyuan Liu <yuanliu@codeaurora.org>