i2c: core: support bus regulator controlling in adapter

Although in the most platforms, the bus power of i2c
are alway on, some platforms disable the i2c bus power
in order to meet low power request.

We get and enable bulk regulator in i2c adapter device.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>