pwm: mediatek: add support for MT7988

MT7988 uses new registers layout just like MT7981 but it supports 8 PWM
interfaces.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240214140454.6438-2-zajec5@gmail.com


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>