soc: mediatek: pwrap: Move PMIC read test sequence in function

The PMIC read test is performed in two places: pwrap_init_dual_io()
and pwrap_init_sidly().
In preparation for adding support for PMICs requiring a companion
part, move this sequence to a new function pwrap_pmic_read_test().

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://lore.kernel.org/r/20230412131216.198313-3-angelogioacchino.delregno@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>