i2c: Use rwsem instead of mutex for board info

By using rwsem we can easily manage recursive calls of
i2c_scan_static_board_info() function without breaking the locking.

Signed-off-by: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

diff --cc drivers/i2c/i2c-boardinfo.c
Simple merge

diff --cc drivers/i2c/i2c-core.c
Simple merge

diff --cc drivers/i2c/i2c-core.h
Simple merge