clk: si5351: make clk-si5351 depend on CONFIG_OF

Calling clk-si5351 driver non-OF ready was too early. This patch
makes clk-si5351 depend on CONFIG_OF again, until things get sorted out.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: fixed spelling of Stephen's name]