fs/fs.c: do_fsload: measure throughput

This patch adds time measurement and throughput calculation for
all supported load commands.

The output of ext2load changes from

---8<---
1830666 bytes read
--->8---

to

---8<---
1830666 bytes read in 237 ms (7.4 MiB/s)
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
[agust: rebased and revised commit log]
Signed-off-by: Anatolij Gustschin <agust@denx.de>