doc: capsule: Document the new mechanism to embed ESL file into dtb

Update the document to specify how the EFI Signature List(ESL) file
can be embedded into the platform's dtb as part of the U-Boot build.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>