net: dsa: microchip: add ksz_prmw32() helper

This will be used in a subsequent patch fixing an errata for writes to
certain PHY registers.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Link: https://lore.kernel.org/r/20230620113855.733526-3-linux@rasmusvillemoes.dk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>