media: rkvdec: Switch to using structs instead of writel

In an effort to merge the rkvdec2 driver [1] with this one, switch from
writel() calls to using structs to represent the register mappings.

This is done in order to have all supported decoders use the same format
in the future and ease reading of the code.

Using structs also improves stability as the hardware is tested and
validated downstream using a similar method.
It was noticed, on decoders, that:
 - Some registers require to be writen in increasing order [2]
 - Some registers, even if unrelated, need to be written to their reset
   values (it was the case here for axi_ddr_[rw]data).

Using structs can also help improving performance later when, e.g.
multicore support is added on RK3588.

[1]: https://lore.kernel.org/all/20250325213303.826925-1-detlev.casanova@collabora.com/
[2]: https://lore.kernel.org/all/20200127143009.15677-5-andrzej.p@collabora.com/



Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>