arm/traps: fix bug in dump_guest_s1_walk handling of level 2 page tables

arm/traps: fix bug in dump_guest_s1_walk handling of level 2 page tables

dump_guest_s1_walk intends to walk to level 2 page table entries but
was failing to do so because of a check that caused level 2 page table
descriptors to be ignored. This change fixes the check so that level 2
page table walks occur as intended by ignoring descriptors unless their
low two bits match the expected sequence [0,1].

For more information, see the ARMv7-A ARM DDI 0406C.b, section B3.5.1.

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