flash: Enhance 'flash read' to read a block of data

It is annoying to read just a single word at a time. Update this
command to print any amount of data. This uses byte format at present.

We could perhaps support something like:

   flash read.8
   flash read.16
   flash read.32

to chose the size.

Example output (with line breaks to keep within git-style limit):

$ flash read FLASH_CTRL 0 20
00000000: 20 25 00 20 1d 26 00 08  69 68 00 08 45 26 00 08
   | %. .&.. ih..E&..|
00000010: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
   |........ ........|

Signed-off-by: Simon Glass <sjg@chromium.org>