arm: Correctly account for PMD in mem_text_address_writeable

Per the comment in __map_init_section:

"In classic MMU format, puds and pmds are folded in to
 the pgds. pmd_offset gives the PGD entry. PGDs refer to a
 group of L1 entries making up one logical pointer to
 an L2 table (2MB), where as PMDs refer to the individual
 L1 entries (1MB). Hence increment to get the correct
 offset for odd 1MB sections."

mem_text_address_writeable accounts for this correctly. On systems
that use the extended page table format, PMDs take up the full
2MB so we should not account for this in figuring out which PMD
to modify. Adjust this as such in mem_text_address_writeable.

Change-Id: I45056dede2f75d32bb16584314cb1d3ea0572a8d
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>