clk: rockchip: simplify rockchip_clk_add_lookup

rockchip_clk_add_lookup is only called from within the file,
so it can be made static. The additional checks are removed
with the following reasoning:

1. The data structure is initialized by rockchip_clk_init(),
   which is called by all rockchip platforms before the clocks
   are registered. Not doing so would result in an incomplete
   clock tree at the moment, which is a fatal error. In other
   parts of the kernel these kind of checks are usually
   omitted, so this was done here. The alternative is adding
   a pr_err to inform the kernel programmer adding a new platform
   about his incorrect code. Apart from that we are also not
   checking if the clock id is within the array boundings.

2. While not used so far by any rockchip platform, 0 is a valid
   clock identifier. To align rockchip closer to other ARM
   platforms we will start using it with rk3588.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20221018151407.63395-8-sebastian.reichel@collabora.com


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