fsi: Move defines to common header

The FSI master registers are common to the hub and AST2600 master (and
the FSP2, if someone was to upstream a driver for that).

Add defines to the fsi-master.h header, and introduce headings to
delineate the existing low level details.

Acked-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>