power: supply: bq24257_charger: Replace depends on REGMAP_I2C with select

regmap is a library function that gets selected by drivers that need
it. No driver modules should depend on it. Depending on REGMAP_I2C makes
this driver only build if another driver already selected REGMAP_I2C,
as the symbol can't be selected through the menu kernel configuration.

Fixes: 2219a935 ("power_supply: Add TI BQ24257 charger driver")
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>