SAMA5 BootROM passes information about the boot source in the r4
register. Add functions to parse these. To make use of this, entry point
must back up the r4 register, because otherwise it's clobbered by local
variable use.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>