drivers: arcv2_timer0: back to use level triggered irq

the pulse triggered timer irq doesn't work for all targets. In
iotdk, we found the clear of IP bit will clear int request
when elapsed called in thread context. So come back to level
triggered way which is supported in all targets, and use the sw
triggered irq to remember the irq request which may be cleared
in non timer int handler.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>