usb: musb: make fifo support configurable

usb: musb: make fifo support configurable

The dynamic FIFO handling under MUSB is optional, and some parts (like
the Blackfin processor) do not implement support for it.

Due to this, the FIFO reading/writing steps need special handling, so
mark the common versions weak so drivers can override.

Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Remy Bohmer <linux@bohmer.net>