clk: fix __clk_init_parent() for single parent clocks

Before commit b3d192d5121f ("clk: simplify __clk_init_parent()"),
__clk_init_parent() called .get_parent() only for multi-parent
clocks.  That commit changed the behavior to call .get_parent()
if available even for single-parent clocks and root clocks.

It turned out a problem because there are some single-parent clocks
that implement .get_parent() callback and return non-zero index.
The SOCFPGA clock is the case; the commit broke the SOCFPGA boards.

To keep the original behavior, invoke .get_parent() only when
num_parents is greater than 1.

Fixes: b3d192d5121f ("clk: simplify __clk_init_parent()")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>