ARM: dts: OMAP5+: Add support for bandgap sensor in SPL

Mark bandgap node as uboot,dm-spl so that it can be accessed in spl

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>