ARM: mm: Get rid of L_PTE_USER ref from PAGE_S2_DEVICE

THe L_PTE_USER actually has nothing to do with stage 2 mappings and the
L_PTE_S2_RDWR value sets the readable bit, which was what L_PTE_USER
was used for before proper handling of stage 2 memory defines.

Changelog:
 [v3]: Drop call to kvm_set_s2pte_writable in mmu.c
 [v2]: Change default mappings to be r/w instead of r/o, as per Marc
       Zyngier's suggestion.

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