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_RDONLY value sets the readable bit, which was what L_PTE_USER
was used for before proper handling of stage 2 memory defines.

Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>