clk: thead: th1520-ap: describe gate clocks with clk_gate

Similar to previous situation of mux clocks, the gate clocks of
clk-th1520-ap drivers are also using a helper that creates a temporary
struct clk_hw and abandons the struct clk_hw in struct ccu_common, which
prevents clock gates to be clock parents.

Do the similar refactor of dropping struct ccu_common and directly use
struct clk_gate here.

This patch mimics the refactor done on struct ccu_mux in 54edba9
("clk: thead: th1520-ap: Describe mux clocks with clk_mux").

Fixes: ae81b69 ("clk: thead: Add support for T-Head TH1520 AP_SUBSYS clocks")
Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Signed-off-by: Drew Fustini <fustini@kernel.org>

Author: Icenowy