intrng: change multi-interrupt root support type to enum

uint32_t is handy for directly interfacing with assembly-language. For
the C portion, enum is much handier. In particular there is no need to
count the number of roots by hand. This also works better for being
able to build kernels with varying numbers of roots.

Switch to INTR_ROOT_COUNT as this better matches the purpose of the
value. Switch to root_type, rather than rootnum for similar reasons.

Remove the default from the core. Better to require the architectures
to declare the type since they will routinely deviate and a default
chosen now will likely be suboptimal.

Leave intr_irq_handler() taking a register type as that better matches
for interfacing with assembly-language.