ASoC: WCD9310: Enable AUX PGA loopback support

Aux PGA (Auxiliary Programmable Gain Amplifier) interface on the
audio codec is used to loopback the analog audio signal from
input (line-in) to output (line-out/HPH/EAR) only through the
analog domain of the codec. AUX PGA also supports analog gain,
which is used to control the gain of the input signal

For WCD9310, line-in5 is used as AUX PGA Left and line-in6 is
used as AUX PGA Right. Add mixer controls to enable routing
AUX PGA inputs to all possible outputs

Change-Id: I85a7865528aa3b2455c190b2617754fe22f3d24a
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>