net: dsa: microchip: do per-port Gbit detection instead of per-chip

KSZ8563 has two 100Mbit PHYs and CPU port with RGMII support. Since
1000Mbit configuration for the RGMII capable MAC is present, we should
use per port validation.

As main part of migration to per-port validation we need to rework
ksz9477_switch_init() function. Which is using undocumented
REG_GLOBAL_OPTIONS register to detect per-chip Gbit support. So, it is
related to some sort of risk for regressions.

To reduce this risk I compared the code with publicly available
documentations. This function will executed on following currently
supported chips:
struct ksz_chip_data            OF compatible
KSZ9477				KSZ9477
KSZ9897				KSZ9897
KSZ9893				KSZ9893, KSZ9563
KSZ8563				KSZ8563
KSZ9567				KSZ9567

Only KSZ9893, KSZ9563, KSZ8563 document existence of 0xf ==
REG_GLOBAL_OPTIONS register with bit field description "SKU ID":
KSZ9893 0x0C
KSZ9563 0x1C
KSZ8563 0x3C

The existence of hidden flags is not documented.

KSZ9477, KSZ9897, KSZ9567 do not document this register at all.

Only KSZ8563 is documented as non Gbit chip: 100Mbit PHYs and RGMII CPU
port. So, this change should not introduce a regression for
configurations with properly used OF compatibles.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>