opensbi: add package for RISC-V

opensbi: add package for RISC-V

OpenSBI is a form of a first-stage bootloader, which initializes
certain parts of an SoC and then passes on control to the second
stage bootloader i.e. an u-boot image.

We're introducing the package with release v1.2, which provides
SBI v0.3 and the SBI SRST extensions which helps to gracefully
reboot/shutdown various HiFive-U SoCs.

Tested on SiFive Unleashed and Unmatched boards.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>