efi/fake_mem: Refactor main two code chunks into functions

There is a whole load of generic EFI memory map code inside of the
fake_mem driver which is better suited to being grouped with the rest
of the generic EFI code for manipulating EFI memory maps.

In preparation for that, this patch refactors the core code, so that
it's possible to move entire functions later.

Tested-by: Dave Young <dyoung@redhat.com> [kexec/kdump]
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> [arm]
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Peter Jones <pjones@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>