cmd: crosec: Move cros_ec_decode_region helper to cmd/cros_ec.c

The cros_ec_decode_region() function is only used in combination
with the crosec cmds. Move the function to the correct place.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: u-boot@lists.denx.de
Acked-by: Simon Glass <sjg@chromium.org>