net: dsa: lantiq_gswip: convert accessors to use regmap

net: dsa: lantiq_gswip: convert accessors to use regmap

Use regmap for register access in preparation for supporting the MaxLinear
GSW1xx family of switches connected via MDIO or SPI.
Rewrite the existing accessor read-poll-timeout functions to use calls to
the regmap API for now.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Acked-by; Hauke Mehrtens <hauke@hauke-m.de>:
Acked-by; Hauke Mehrtens <hauke@hauke-m.de>:
Link: https://patch.msgid.link/535d968bc6319a74bdf76166ef19364ee659285f.1761045000.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>