disk: part_efi: fix **pgpt_pte == NULL

Code was setting **pgpt_pte == NULL, which meant that the pointer
to the gpt_pte would be stored at RAM address 00000000. This 'worked'
on T20 (SDRAM starts @ 0x00000000), but hung gpt/EFI access on T30
(SDRAM starts @ 0x80000000).

Signed-off-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Doug Anderson <dianders@chromium.org>