AXI performance monitor driver (#2222)

Uses the debugfs I/F to provide access to the AXI
bus performance monitors.

Requires the new mailbox peripheral access for access
to the VPU performance registers, system bus access
is done using direct register reads.

Signed-off-by: James Hughes <james.hughes@raspberrypi.org>

raspberrypi_axi_monitor: suppress warning

Suppress the following warning by casting the pointer to and uintptr_t
before to u32:

Signed-off-by: Matteo Croce <mcroce@redhat.com>