arm/traps: fix bug in dump_guest_s1_walk L1 page table offset computation

arm/traps: fix bug in dump_guest_s1_walk L1 page table offset computation

The dump_guest_s1_walk function was incorrectly using the top 10 bits of
the virtual address to select the L1 page table index.  The correct
amount is 12 bits, resulting in a shift of 20 bits rather than 22.

For more details, see the ARMv7-A ARM DDI 0406C.b, section B3.5,
"Short-descriptor translation table format."

Signed-off-by: Jonathan Daugherty <jtd@galois.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>