dts: renamed FIT_EMBED to MULTI_DTB_FIT and moved it to the dts Kconfig

CONFIG_FIT_EMBED might be confused with CONFIG_OF_EMBED, rename it
MULTI_DTB_FIT as it is able to get a DTB from a FIT image containing
multiple DTBs. Also move the option to the Kconfig dedicated to the DTS
options and create a README for this feature.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>