ASoC: wcd9304: enable charge pump for headphones

It was found that headphone output level is extremely
quiet even if gain on both digital and analog domain have
been maxed out. The problem was that charge pump is not
enabled.

Change-Id: I1877abe547d2b666151c9b71289824d54c60092d
CRs-fixed: 361008
Signed-off-by: Patrick Lai <plai@codeaurora.org>