clk: Add error handling to clk_fetch_parent_index()

There are at least two different error cases that can happen in
clk_fetch_parent_index() function:
 - allocation failure,
 - parent clock lookup failure,
however it returns only an u8, which is supposed to contain parent clock
index.

This patch modified the function to return full int instead allowing
positive clock indices and negative error codes to be returned. All
users of this function are adjusted as well to handle the return value
correctly.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>