leds: bd2802: Convert to use GPIO descriptors

The Rohm BD2802 have no in-kernel users so we can drop the
GPIO number from the platform data and require users to
provide the GPIO line using machine descriptors.

As the descriptors come with inherent polarity inversion
semantics, we invert the calls to set the GPIO line such
that 0 means "unasserted" and 1 means "asserted".

Put a note in the driver that machine descriptor tables
will need to specify that the line is active low.

Cc: Kim Kyuwon <chammoru@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>