cmd_mtdparts: use 64 bits for flash size, partition size & offset

This matches the 64 bit size in struct mtd_info and allows the mtdparts
command to function correctly with a flash >= 4GiB. Format specifiers
for size & offset are given the ll length, matching its use in
drivers/mtd in absence of something like inttypes.h/PRIx64.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Stefan Roese <sr@denx.de>