[PATCH] RISC-V: Resurrect the MMIO timer implementation for M-mode systems

[Christoph Hellwig]

I won't have time to set up the K210, but this fixes the timer issues
on older Qemu versions that don't implement the time CSR.  Although I
also need to revert the binfmt_flat patch to actually get to userspace.

Tested-by: Christoph Hellwig <hch at lst.de>