mfd: Fix twl4030-core.c build error

This is a fix for:

twl4030-core.c:(.text+0x16a797): undefined reference to `clk_get_rate'
twl4030-core.c:(.text+0x16a797): undefined reference to `clk_put'

on x86 and x86_64, as the clock API is not defined on those platforms.

Signed-off-by: Samuel Ortiz <sameo@openedhand.com>