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 hifive serial and pwm drivers

This implements the required drivers to pass sel4test