ARM: KVM: fix L_PTE_S2_RDWR to actually be Read/Write

Looks like our L_PTE_S2_RDWR definition is slightly wrong,
and is actually write only (see ARM ARM Table B3-9, Stage 2 control
of access permissions). Didn't make a difference for normal pages,
as we OR the flags together, but I'm still wondering how it worked
for Stage-2 mapped devices, such as the GIC.

Brown paper bag time, again.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>