drivers: eth: dsa_ksz8xxx: Add support for KSZ8463/KSZ8463F

This commit adds basic support for KSZ8463/KSZ8463F chips to the
dsa_ksz8xxx.c driver.

These chips have limited register compatibility with other members
of the KSZ8XXX family - their registers are 16 bits wide as opposed
to the 8-bit registers supported by the driver for KSZ8794 and
KSZ8863. Following the general logic of the existing code,
the 16-bit registers of KSZ8463 are split into 8-bit halves.

For the KSZ8463F chip, it is assumed that both ports are used
in Fiber mode.

A new configuration option, CONFIG_DSA_KSZ_PORT_ISOLATING, has been
added to isolate traffic between DSA slave ports.

The driver has been tested on a custom board with an STM32F7 SoC.

Signed-off-by: Aleksandr Senin <al@meshium.net>