cmd_sf: Add print mesgs on sf read/write commands

This patch adds a print messages while using 'sf read' and
'sf write' commands to make sure that how many bytes read/written
from/into flash device.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Acked-by: Tom Rini <trini@ti.com>