The variation in access size requirements and spacing of registers of

The variation in access size requirements and spacing of registers of
NS16550 clones is a bit of a mess.  There are FDT properties that describe
these but these proprties are optional for some variants and the default
values depend on the compatible string being used.  Most variants that have
shown up in the ARM world use 32-bit access and spacing.  But true NS16550A
implementations of course use 8-bit access and spacing.  So look at the
compatible strings to set up the appropriate defaults.  Makes the "true"
NS16550A implemented by QEMU work for RISCV targets.

ok jsg@, drahn@