net: pcs: rzn1-miic: Convert to for_each_available_child_of_node() helper

Simplify miic_parse_dt() by using the for_each_available_child_of_node()
helper instead of manually skipping unavailable child nodes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/3e394d4cf8204bcf17b184bfda474085aa8ed0dd.1738771631.git.geert+renesas@glider.be


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