power: supply: bq27xxx: add separate flag for single SoC register

bq27000, bq27010 and upcoming bq34z100 have a single byte SoC
register.  However except this similarity, bq34z100 is quite different
than bq27000/bq27010, so flag BQ27XXX_O_ZERO cannot be reused here.  Add
a new bit flag describing that SoC is a single byte register.

No functional change for bq27000 and bq27010.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>