clock-gcc-8916: Get all external clocks before registration

clock-gcc-8916 only gets parent for xo_a_clk_src after registering
its clock table with framework. This introduces two errors:
1) The parent of xo_a_clk_src is potentially not handed off
2) When clock table registers, xo_a_clk_src fails handoff because
it's an external clock without a parent. Thus its CLKFLAG_INIT_DONE
is not set even if its parent is actually ready. Instead,
xo_a_clk_src is added onto orphan list.

CLKFLAG_INIT_DONE will only be set when another driver registers
clocks and triggers a handoff for orphan clocks.

Fix the issue by filling the parent before registering clock table
in clock-gcc-8916.

Change-Id: Ie0f568cd530a2c4b218c7db8094b1bd10196a10f
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>