vchiq: fix compilation error on armv7 in CI and local armv7 builds

This happens in CI and on local armv7 builds, but not when I cross
compile to armv7 locally.

/usr/src/sys/contrib/vchiq/interface/vchiq_arm/vchiq_2835_arm.c:549:71: error: format specifies type 'unsigned int' but the argument has type 'unsigned long' [-Werror,-Wformat]

549 |                 "free_pagelist - %zx, %d (%u bytes @%p)", (size_t)pagelist, actual, pagelist->length, bi->buf);

So just be very specific about the printf types and explicitly
cast things.