riscv: Add a reset_cpu() function

riscv: Add a reset_cpu() function

The current do_reset() is called from a command context. Add a function
which can be used from anywhere, as is done on ARM. Adjust do_reset()
to call it.

Note that reset_cpu() is normally provided by SYSRESET so make this
declaration conditional on that being disabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Tested-by: Chanho Park <chanho61.park@samsung.com>