trivial: Fix compiler warnings

- Mark unused variables unused
- Initialize uninitialized variables

libplatsupport: fix previous reverts

Due to my messing up of changelists, the revert
did not go through as cleanly as I'd hoped.

This commit reinstates
"libplatsupport: riscv plic irqchip parser"
23092562396ae9209679c83d75d25a953b70c2c6
and picks changes necessary from
"libplatsupport: hifive ltimer refactor"
c0c8ddcfac95a09b50e82cfa88367145d7aeaa5e
in order to build
"libplatsupport: refactor more timers"
ef1dfb7da52b8324281ee6999951bf603d44bdd7

Revert "libplatsupport: hifive ltimer refactor"

This reverts commit c0c8ddcfac95a09b50e82cfa88367145d7aeaa5e.

libplatsupport: hifive ltimer refactor

- Introduce two new helper functions
`helper_fdt_alloc_simple`, `create_ltimer_simple`
These are only used when there's 1 reg and 1 irq
to register.
- Refactor hifive timers to use these helper functions.
- Nit: init time_h explicitly instead of being calloc'd

libplatsupport: Add common timer IRQ callback note

This commit adds documentation to the common timer IRQ callback function
to state that it assumes that interrupts are level triggered.

libplatsupport: Refactor the internal handle_irq

Now that the handle_irq functions are no longer externally visible, this
commits refactors the handle_irq functions so that they can be accessed
internally. Also add logic to invoke the user supplied callbacks after
handling the timer IRQ.

libplatsupport: Add IRQ callback helper for ltimer

This commit introduces a new struct type and function that is intended
to be passed to the IRQ interface as a callback for timer interrupts.