SoC: core: Introduce macro SOC_SINGLE_MULTI_EXT

For some of the mixer controls, client would like to register
count as a parameter. Macro adds support to specify the count.

Change-Id: I26343af2e4a35952d93dc2d5fa0caad2a3e50ffe
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>
Signed-off-by: Meng Wang <mwang@codeaurora.org>