at91: use C structs for AT91 OHCI code

This patch is part of migrating the AT91 support towards
using C struct for all SOC access.

It removes one more CONFIG_AT91_LEGACY warning.

at91_pmc.h needs cleanup after migration of the drivers
has been done.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>