ACPI / APEI: Split fixmap pages for arm64 NMI-like notifications

Now that ghes uses the fixmap addresses and locks via some indirection
we can support multiple NMI-like notifications on arm64.

These should be named after their notification method. x86's
NOTIFY_NMI can stay as it is, change the SEA fixmap entry to use
FIX_APEI_GHES_SEA.

Future patches can add support for FIX_APEI_GHES_SEI and
FIX_APEI_GHES_SDEI_{NORMAL,CRITICAL}.

Signed-off-by: James Morse <james.morse@arm.com>