spl: riscv: add os type for next booting stage

If SPL_LOAD_FIT_OPENSBI_OS_BOOT is enabled, the function
spl_invoke_opensbi should change the target OS type to IH_OS_LINUX.
OpenSBI will load the Linux image as the next boot stage.
The os_takes_devicetree function returns a value of true or false
depending on whether or not SPL_LOAD_FIT_OPENSBI_OS_BOOT is enabled.

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