net: dsa: mv88e6xxx: add Amethyst specific SMI GPIO function

The existing mv88e6390_g2_scratch_gpio_set_smi() cannot be used on the
88E6393X as it requires certain P0_MODE, it also checks the CPU mode
as it impacts the bit setting value.

This is all irrelevant for Amethyst (MV88E6191X/6193X/6393X) as only
the default value of the SMI_PHY Config bit is set to CPU_MGD bootstrap
pin value but it can be changed without restrictions so that GPIO pins
9 and 10 are used as SMI pins.

So, introduce Amethyst specific function and call that if the Amethyst
family wants to setup the external PHY.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>