i2c: Make i2c_register_board_info() a NOP when CONFIG_I2C_BOARDINFO=n

Don't require platform code to be #ifdeffed according to whether
I2C is enabled or not ... if it's not enabled, let GCC compile out
all I2C device declarations.  (Issue noted on an NSLU2 build that
didn't configure I2C.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>