clk: thead: Port clock controller driver of TH1520 SoC

clk: thead: Port clock controller driver of TH1520 SoC

The driver is adapted from Linux kernel's version of clk-th1520-ap.c,
with only output clocks for external sensors, which are barely useful in
bootloaders, removed.

Same as the mainline driver, it currently lacks of ability to enable and
reconfigure PLLs, which could be implemented later.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>