riscv: Move timer portions of SiFive CLINT to drivers/timer

Half of this driver is a DM-based timer driver, and half is RISC-V-specific
IPI code. Move the timer portions in with the other timer drivers. The
KConfig is not moved, since it also enables IPIs. It could also be split
into two configs, but no boards use the timer but not the IPI atm, so I
haven't split it.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>