riscv: dts: binman: add condition for opensbi os boot

Add condition for OpenSBI OS boot mode, by default it is not enabled.
By default, binman creates the output file u-boot.itb.
If SPL_OPENSBI_OS_BOOT is enabled, linux.itb will be created
after compilation instead of the default u-boot.itb.

Signed-off-by: Randolph <randolph@andestech.com>
Reviewed-by: Simon Glass <sjg@chromium.org>