timer: riscv_machine_timer: prevent spurious interrupt while rearming the timer

Rearming the riscv machine timer is done by first updating the
mtimecmp low value register. If the low value is updated with a
relatively small value, a timer interrupt can be generated while
updating the mtimecmp high value.

To avoid such a spurious interrupt to occur, disable the timer
interrupt while rearming the timer.

Change-Id: I50ab3f19554a9a8dfe70943b6da0d20be3de88dc
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>