net: phy: micrel: add support for KSZ886x switches in MIIM mode

This patch adds a phy driver for the Micrel KSZ886x switches.

Similarly to the KSZ8895, SoC MAC is directly connected to the switch
MAC on the switch CPU port, so the link to the switch is always up.

KSZ886x switches can be used in the following configuration modes:
- Unmanaged mode with config stored in external EEPROM
- Managed mode over SPI
- Managed mode over I2C
- Managed mode over mdio/mdc (aka MIIM or SMI)

This patch supports only unmanaged and MIIM modes.

Based on Micrel KSZ886x driver from Linux kernel and
Micrel KSZ8895 driver from U-Boot.

Verified with the KSZ8863MLL.

Signed-off-by: Alexey Firago <alexey_firago@mentor.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>