doc: imx: Move SPD related info to the appropriate doc

Currently the Serial Download Protocol tools and procedure are
documented in two places:

- doc/imx/README.sdp
- doc/imx/README.imx6

It is better to consolidate all SDP related information into
README.sdp file, so move the content from README.imx6 to
README.sdp.

Signed-off-by: Breno Lima <breno.lima@nxp.com>