Add at91sam9260 as part of supporting the PMC bindings for ARM at91.
The file will be wired into the build in a follow-up patch.

The file is a copy from kernel 5.0-rc6 modified to build
with barebox.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>