thermal: power_allocator: round the division when divvying up power

In situations where there is an uneven number of cooling devices, the
division of power among them can lead to a milliwatt being dropped on
the floor due to rounding errors.  This doesn't sound like a lot, but
some devices only grant the lowest cooling device state for their
maximum power.  So for instance, if the granted_power is the maximum
power and all devices are getting their maximum power, one would get
max_power - 1, making it choose cooling device state 1, instead of 0.

Round the division to make the calculation more accurate.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>