lib: utils: fdt_fixup: avoid buffer overrun

fdt_reserved_memory_fixup() uses filtered_order[PMP_COUNT]. The index
must not reach PMP_COUNT.

Fixes: 199189bd

 ("lib: utils: Mark only the largest region as reserved in FDT")
Addresses-Coverity-ID: 1536994 ("Out-of-bounds write")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>