x86: cmd: hob: Fix display of resource type for system memory

The resource type for system memory is currently displayed as
"unknown", which is wrong.

Fixes: 51af144e ("x86: Allow showing details about a HOB entry")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on chromebook_coral, chromebook_samus, chromebook_link, minnowmax
Tested-by: Simon Glass <sjg@chromium.org>