clk: rockchip: Add PLL flag ROCKCHIP_PLL_FIXED_MODE

RK3528 comes with a new PLL variant: its "PPLL", which mainly generates
clocks for the PCIe controller, operates in normal mode only. Let's
describe it with flag ROCKCHIP_PLL_FIXED_MODE and handle it in code.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Link: https://lore.kernel.org/r/20250217061142.38480-7-ziyao@disroot.org


Signed-off-by: Heiko Stuebner <heiko@sntech.de>