The optimal management of VMID depends on a number of factors which vary
dynamically at runtime, for example the connection to a system docking
station. In some circumstances it is desirable to keep VMID enabled all
the time, in others it is desirable to aggressively power it up and down.

Provide a callback allowing machine driver to configure either the normal
power up/down mode (WM8994_VMID_MODE_NORMAL) or to maintain VMID even
when idle (WM8994_VMID_MODE_FORCE). This callback, wm8994_vmid_mode(),
should be called with the CODEC lock.
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>