clk: thead: th1520-ap: Correctly refer the parent of osc_12m

clk: thead: th1520-ap: Correctly refer the parent of osc_12m

The "osc_12m" fixed factor clock refers the external oscillator by
setting clk_parent_data.fw_name to osc_24m, which is obviously wrong
since no clock-names property is allowed for compatible
thead,th1520-clk-ap.

Refer the oscillator as parent by index instead.

Fixes: ae81b69fd2b1 ("clk: thead: Add support for T-Head TH1520 AP_SUBSYS clocks")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Signed-off-by: Drew Fustini <fustini@kernel.org>