arch/risc-v/riscv_mtimer: Update the mtimecmp value once per interrup…

[g2gps]

Summary

Cache the next timeout value in the drivers instance and update the mtimecmp value once. This is advantageous as the opensbi ecall to set the timer is expensive in systems which don't have the supervisor mode timer extension.

Impact

Mild reduction in the mtimer interrupt time, when CONFIG_SCHED_TICKLESS is used, on the Litex platform.

Before

IRQ HANDLER  ARGUMENT    COUNT    RATE (s) TOTAL TIME(us) MEAN TIME(us) MAX TIME(us) 
21   40015396  40434f10        1204        12.607                   162752                     135                    463  // os test
21   40015396  40434f10        2669        850.426                  580978                     217                    718  // custom application

With this change

IRQ HANDLER  ARGUMENT    COUNT    RATE (s) TOTAL TIME(us) MEAN TIME(us) MAX TIME(us) 
21   40015396       40434f10       1217       11.898                142299                     116                   390  // os test
21   40015396       40434f10       2043     808.788                   372405                     182                   619 // custom application

Testing

ostest_knsh32_new.txt ostest_knsh32_old.txt ostest_knsh64_new.txt ostest_knsh64_old.txt ostest_litex.txt

Note: OS test on qemu32 and qemu64 both return -1 before and after this change.

It would be great if this change could be tested on another hardware platform.