sound: tas2557: set FAST mode for VBOB in requested case

Some capacitance on VBOB power rail as causing a buzzing sound.
VBOB is by default in AUTO mode and alternate between PFM and PWM
modes based on the rail load. Forcing rail to PWM reduces the buzzing
sound but is less efficient at low current than PFM so this patch
will force VBOB to FAST mode (PWM) only on the selected route
configuration set by device tree.

This patch also allows FAST mode to be requested on the VBOB regulator.

Bug: 64839820
Change-Id: I965269ce9ffde33e017090e776c003db81533959
Signed-off-by: Thierry Strudel <tstrudel@google.com>