floppy: add references to 82077's extra registers

This controller provides extra status registers SRA and SRB as well
as a tape drive register (TDR) and a data rate select register (DSR),
which are referenced in the sparc port, so let's have their symbolic
definitions centralized.

Link: https://lore.kernel.org/r/20200331094054.24441-3-w@1wt.eu


Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Denis Efremov <efremov@linux.com>