spl: support booting via RISC-V OpenSBI

spl: support booting via RISC-V OpenSBI

RISC-V OpenSBI is an open-source implementation of the RISC-V Supervisor
Binary Interface (SBI) specification. It is required by Linux and U-Boot
running in supervisor mode. This patch adds support for booting via the
OpenSBI FW_DYNAMIC firmware. It supports OpenSBI version 0.4 and higher.

In this configuration, U-Boot SPL starts in machine mode. After loading
OpenSBI and U-Boot proper, it will start OpenSBI. All necessary
parameters are generated by U-Boot SPL and are passed to OpenSBI. U-Boot
proper is started in supervisor mode by OpenSBI. Support for OpenSBI is
enabled with CONFIG_SPL_OPENSBI. An additional configuration entry,
CONFIG_SPL_OPENSBI_LOAD_ADDR, is used to specify the load address of the
OpenSBI firmware binary. It is not used directly in U-Boot and instead
is intended to make the value available to scripts such as FIT
configuration generators.

The header file include/opensbi.h is based on header files from the
OpenSBI project. They are recent, as of commit bae54f764570 ("firmware:
Add fw_dynamic firmware").

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>