i2c: break out slave support into separate file

Also removes some ifdeffery.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
diff --git a/drivers/i2c/i2c-core.h b/drivers/i2c/i2c-core.h
index 17700bf..77c22b0 100644
--- a/drivers/i2c/i2c-core.h
+++ b/drivers/i2c/i2c-core.h
@@ -27,3 +27,4 @@
 extern struct list_head	__i2c_board_list;
 extern int		__i2c_first_dynamic_bus_num;
 
+int i2c_check_7bit_addr_validity_strict(unsigned short addr);