USB: musb: add notes for Blackfin anomalies

Add some helpful notes about how the driver works around different
anomalies that exist in the on-chip host controller.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>