acpi, apei, ghes: Factor out ioremap virtual memory for IRQ and NMI context.

GHES currently maps two pages with atomic_ioremap.  From now
on, NMI is architectural depended so there is no need to allocate
an NMI page for platforms without NMI support.

To make it possible to not use a second page, swap the existing
page order so that the IRQ context page is first, and the optional
NMI context page is second.  Then, use HAVE_ACPI_APEI_NMI to decide
how many pages are to be allocated.

Signed-off-by: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>