riscv timer: remove intermediate helper

get_counts() doesn't do anything at the moment but return the result of
get_cycles(), so remove it.

For clarity, rename get_cycles() to get_timecount(); RISC-V defines
separate time and cyclecount CSRs, so let's avoid confusing the two.
They may be backed by the same underlying clock, but this is an
implementation detail.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35461

(cherry picked from commit b82f4170fca86e9b41b613715852d4f225b26e3a)