regulator: sy8824x: Enable REGCACHE_FLAT

Enable regmap cache to reduce i2c transactions and corresponding
interrupts if regulator is accessed frequently. Since the register map
is small -- there's only one register in sy8824c and sy8824e, there
are only two registers in sy20276 and sy20278, so we use a FLAT regmap
cache.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20210803165043.042ec24d@xhacker.debian


Signed-off-by: Mark Brown <broonie@kernel.org>