[IA64] kexec: add __va_efi

[IA64] kexec: add __va_efi

sal_desc_entry_point() converts physical addresses into virtual
addresses using __va() and these virtual addresses are used
to register the SAL and PAL handlers - which exist in firmware
memory.

As the mapping of firmware memory is being moved from
a PAGE_OFFSET of (0xf << 60) to a PAGE_OFFSET of (0xe << 60),
__va() does not provide the correct virtual address.

By adding __va_efi(), which uses EFI_PAGE_OFFSET=(0xe << 60),
and using this in sal_desc_entry_point(), the correct address is
used.

On an HP rx2600 this eliminates the problem where the SAL rendezvous address
can't be registered and subsequently the boot fails. I suspect it
solves similar problems that have been seen on other HP machines too.

Actually, its rather amazing that the HP rx2620 and Tiger2 that
I have been using work without this fix.

Signed-off-by: Simon Horman <horms@verge.net.au>