cmd: replace the cast of the memory access to a fixed bit type in itest

This patch fixes a bug that long word(.l) memory access in 'itest'
command reads the 8bytes of the actual memory on 64-bit architecture.
The cast to the memory pointer should use a fixed bit type.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>