i2c: fix build bug in i2c-designware.c

This build error triggers on x86:

 drivers/built-in.o: In function `i2c_dw_init':
 i2c-designware.c:(.text+0x4e37ca): undefined reference to `clk_get_rate'
 drivers/built-in.o: In function `dw_i2c_probe':
 i2c-designware.c:(.devinit.text+0x51f5e): undefined reference to `clk_get'
 i2c-designware.c:(.devinit.text+0x51f76): undefined reference to `clk_enable'
 i2c-designware.c:(.devinit.text+0x520ff): undefined reference to `clk_disable'
 i2c-designware.c:(.devinit.text+0x52108): undefined reference to `clk_put'

Because this new driver uses the clk_*() facilities which is an
ARM-only thing currently.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>