binman: etype: Add u-boot-spl-pubkey-dtb etype

This adds a new etype 'u-boot-spl-pubkey-dtb'. The etype adds the public
key from a certificate to the dtb. This creates a '/signature' node which
is turn contains the fields which make up the public key. Usually this
is done by 'mkimage -K'. However, 'binman sign' does not add the public
key to the SPL. This is why the pubkey is added using this etype.

The etype calls the underlying 'fdt_add_pubkey' tool.

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>