soc: mediatek: mtk-svs: Use bitfield access macros where possible

In order to enhance readability and safety during registers setup
and value retrieval, redefine a few register related macros and
convert all open-coded instances of bitfield setting/retrieval
to use the FIELD_PREP() and FIELD_GET() macros.
While at it, some macros were renamed to further enhance readability.

This commit brings no functional changes.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://lore.kernel.org/r/20220825184616.2118870-3-nfraprado@collabora.comSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>