m68k: Add infrastructure for machine-specific random_get_entropy()

On m68k, get_cycles() (the default implementation for random_get_entropy())
always returns zero, providing no entropy for the random driver.

Add a hook where platforms can provide their own implementation, and wire
it up in the infrastructure provided by commit
61875f30 ("random: allow architectures to
optionally define random_get_entropy()").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>