aspeed/smc: Introduce a new addr_width() class handler

The AST2400 SPI controller has a transitional HW interface and it
stores the address width currently in use in a different register than
all the other SMC controllers. It needs special handling when working
in 4B mode.

Make it clear through a class handler. This also removes another use
of the segments array.

Signed-off-by: Cédric Le Goater <clg@kaod.org>