spl: Add support for CONFIG_OF_EMBED=y

spl: Add support for CONFIG_OF_EMBED=y

CONFIG_OF_EMBED=y is the option which is here only for testing purpose
and shouldn't be enabled by default as is describe at:
"dts: Add a comment about CONFIG_OF_EMBED being for local use"
(sha1: 3d3f60cb7a6bb6c338e00a9769fa918a8536096c)

But still enabling this option locally shouldn't end up with compilation
error when you build SPL. This patch fix it.

Compilation error:
lib/built-in.o: In function `fdtdec_setup':
/mnt/disk/u-boot/lib/fdtdec.c:1246: undefined reference to
`__dtb_dt_begin'

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reported-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>