tools/xcutils/readnotes: adjust print_l1_mfn_valid_note

tools/xcutils/readnotes: adjust print_l1_mfn_valid_note

Use the new PTRVAL macros and elf_access_unsigned in
print_l1_mfn_valid_note.

No functional change unless the input is wrong, or we are reading a
file for a different endianness.

Separated out from the previous patch because this change does produce
a difference in the generated code.

This is part of the fix to a security issue, XSA-55.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Chuck Anderson <chuck.anderson@oracle.com>
v2: Split out into its own patch.