arm64: booting: Require placement within 48-bit addressable memory

Some configurations (i.e., 64k + LVA/LPA) can tolerate a physical
placement of the kernel image outside of the 48-bit addressable region,
but given that the loader has no way of knowing whether or not the image
in question supports LVA/LPA, it currently has no choice but to place it
below the 48-bit mark.

Once we add support for LPA2, which allows 52-bit physical and virtual
addressing when using 4k or 16k pages, but in way that relies on
increasing the number of paging levels, there will be more variety in
the configurations that may or may not support this.

So redefine bit #3 in the Image header as 'must be placed within 48-bit
addressable memory', as this is the current de facto meaning.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20221122170249.2453853-1-ardb@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

Documentation/arm64/booting.rst

@@ -121,8 +121,9 @@ Header notes:
 			  to the base of DRAM, since memory below it is not
 			  accessible via the linear mapping
 			1
-			  2MB aligned base may be anywhere in physical
-			  memory
+			  2MB aligned base such that all image_size bytes
+			  counted from the start of the image are within
+			  the 48-bit addressable range of physical memory
   Bits 4-63	Reserved.
   ============= ===============================================================