riscv: kconfig: introduce SPL_LOAD_FIT_OPENSBI_OS_BOOT symbol

Introduce common Kconfig symbol for riscv architecture.
This symbol SPL_LOAD_FIT_OPENSBI_OS_BOOT is like falcon mode on ARM,
the Falcon boot is a shortcut boot method for SD/eMMC targets. It
skips the loading the RAM version U-Boot. Instead, it will loads
the FIT image and boots directly to Linux.

When SPL_OPENSBI_OS_BOOT is enabled, linux.itb is created after
compilation instead of the default u-boot.itb. It initialises memory
with the U-Boot SPL at the first stage, just as a normal boot process
does at the beginning. Instead of jumping to the U-Boot proper from
OpenSBI before booting the Linux kernel, the RISC-V falcon mode
process jumps directly to the Linux kernel to gain shorter booting time.

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