ARM: pgtable: Fix compiler warning in ioremap.c introduced by nopud

ARM: pgtable: Fix compiler warning in ioremap.c introduced by nopud

With the arch/arm code conversion to pgtable-nopud.h, the section and
supersection (un|re)map code triggers compiler warnings on UP systems.
This is caused by pmd_offset() being given a pgd_t argument rather than
a pud_t one. This patch makes the necessary conversion with the
assumption that the pud is folded into the pgd. The page table setting
code only loops over the pmd which is enough with the classic page
tables. This code is not compiled when LPAE is enabled.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>