dt-bindings: sy8824x: Document SY20278 support

SY20276 is an I2C-controlled adjustable voltage regulator made by
Silergy Corp. The differences between SY8824C and SY20278 are
different regs for mode/enable.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20190827163754.170cf130@xhacker.debian


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