[rpi] remove IRQ support for BCM233x RNG

Upstream DTBs don't provide IRQ lines for the RNG. Moreover, harvesting
bytes as often as the RNG interrupt is triggered (87 times per sec) is an
overkill.

For these reasons, get rid of the interrupt mode and make callout mode the
default, with random bits harvested every 4 seconds.

Submitted by: Sylvain Garrigues <sylgar@gmail.com>
Reviewed by: ian, imp, manu, mmel
Approved by: emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D14541