risc-v/bl808: Implement Timer with OpenSBI

[lupyuen]

Summary

The implementation of the RISC-V Timer for BL808 SoC is incomplete. This PR implements the BL808 RISC-V Timer by calling OpenSBI. The code is derived from NuttX for RISC-V QEMU.

The implementation of up_timer_initialize with OpenSBI is explained in this article.

Impact

With this PR, sleep and other Timer Functions will work correctly on BL808 SoC, instead of hanging indefinitely.

No impact on other SoCs.

Testing

We tested with ostest on Ox64 BL808 SBC:

$ tools/configure.sh ox64:nsh

Before PR

ostest hangs when accessing the RISC-V Timer:

nsh> ostest
...
ostest_main: Started user_main at PID=7
(Hang)

(NuttX Log)

After PR

ostest works correctly with the RISC-V Timer and terminates properly:

nsh> ostest
...
user_main: Exiting
ostest_main: Exiting with status -1
nsh> 

(NuttX Log)

[pkarashchenko]

LGTM!