Make div_round_up() correct for divisors that are not a power of 2

Make div_round_up() correct for divisors that are not a power of 2

The current div_round_up() implementation relies on round_up() which
only works correctly for boundaries that are a power of 2. It is
documented as such, but this still seems dangerously easy to overlook,
especially since many other environments (e.g. the Linux kernel) have a
similar macro without these limitations.

There is a different way to calculate this that can deal with all kinds
of divisors without other drawbacks, so let's just use that instead.

Change-Id: Id382736683f5d4e880ef00c53cfa23a2f9208440
Signed-off-by: Julius Werner <jwerner@chromium.org>