mtd: mtdpart: Change size type from fdt_addr_t to fdt_size_t

Set correct type for 3rd argument of ofnode_get_addr_size_index_notrans()
function. It expects fdt_size_t * and not fdt_addr_t *.

When these two types do not have same size then U-Boot throw compile
warning:

    drivers/mtd/mtdpart.c: In function ‘add_mtd_partitions_of’:
    drivers/mtd/mtdpart.c:906:57: warning: passing argument 3 of ‘ofnode_get_addr_size_index_notrans’ from incompatible pointer type [-Wincompatible-pointer-types]
       offset = ofnode_get_addr_size_index_notrans(child, 0, &size);
                                                             ^~~~~
    In file included from include/dm/device.h:13,
                     from include/linux/mtd/mtd.h:26,
                     from include/ubi_uboot.h:28,
                     from drivers/mtd/mtdpart.c:27:
    include/dm/ofnode.h:530:25: note: expected ‘fdt_size_t *’ {aka ‘long long unsigned int *’} but argument is of type ‘fdt_addr_t *’ {aka ‘long unsigned int *’}
                 fdt_size_t *size);
                 ~~~~~~~~~~~~^~~~

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>