riscv: timer: add timer_get_boot_us for BOOTSTAGE

riscv: timer: add timer_get_boot_us for BOOTSTAGE

timer_get_boot_us function is required to record the boot stages as
us-based timestamp.
To get a micro-second time from a timer tick, this converts the
formula like below to avoid zero result of (tick / rate) part.

From: time(us) = (tick / rate) * 1000000
To  : time(us) = (tick * 1000) / (rate / 1000)

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>