div64.h: optimize do_div() for power-of-two constant divisors

Let's perform the obvious mask and shift operation in this case.

On 32-bit targets, gcc is able to do the same thing with a constant
divisor that happens to be a power of two i.e. it turns the division
into an inline shift, but it doesn't hurt to be explicit.

Signed-off-by: Nicolas Pitre <nico@linaro.org>