net: dsa: microchip: Use regs[] to access REG_PTP_RTC_SUB_NANOSEC

net: dsa: microchip: Use regs[] to access REG_PTP_RTC_SUB_NANOSEC

Accesses to the PTP_RTC_SUB_NANOSEC register are done through a
hardcoded address which doesn't match with the KSZ8463's register
layout.

Add a new entry for the PTP_RTC_SUB_NANOSEC register in the regs[]
tables.
Use the regs[] table to retrieve the PTP_RTC_SUB_NANOSEC register
address when accessing it.
Remove the macro defining the address to prevent further use.

Signed-off-by: Bastien Curutchet (Schneider Electric) <bastien.curutchet@bootlin.com>
Link: https://patch.msgid.link/20260105-ksz-rework-v1-6-a68df7f57375@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>