The hash mmu page fault handling code is responsible for generating ISIs
and DSIs when access permissions cause an access to fail. Part of this
involves setting the srr1 or dsisr registers to indicate what causes the
access to fail. Add defines for the bit fields of these registers and
rework the code to use these new defines in order to improve readability
and code clarity.

While we're here, update what is logged when an access fails to include
information as to what caused to access to fail for debug purposes.
Signed-off-by: NSuraj Jitindar Singh <sjitindarsingh@gmail.com>
[dwg: Moved constants to cpu.h since they're not MMUv3 specific]
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>