ARM: OMAP2+: AM43x: determine features

Determine AM43x device features by reusing AM335x helper as feature
register layout is similar.
And also exporting AM43xx family name.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>