firmware: arm_sdei: Add ACPI GHES registration helper

APEI's Generic Hardware Error Source structures do not describe
whether the SDEI event is shared or private, as this information is
discoverable via the API.

GHES needs to know whether an event is normal or critical to avoid
sharing locks or fixmap entries, but GHES shouldn't have to know about
the SDEI API.

Add a helper to register the GHES using the appropriate normal or
critical callback.

Signed-off-by: James Morse <james.morse@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>

---
Changes since v4:
 * Moved normal/critical callbacks into the helper, as APEI needs to know.
 * Tinkered with the commit message.
 * Dropped Punit's Reviewed-by.

Changes since v3:
 * Removed acpi_disabled() checks that aren't necessary after v2s #ifdef
   change.

Changes since v2:
 * Added header file, thanks kbuild-robot!
 * changed ifdef to the GHES version to match the fixmap definition

Changes since v1:
 * ghes->fixmap_idx variable rename