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: Reset hifive pwm prescaler

The prescaler was not reset each time a new timeout was set. The bitwise
OR operation would take the previous prescaler bits and mess up the
prescaler setting for the new timeout. This commit resets the prescaler
so that the timeouts will now work as intended.

hifive: Use interrupt bits to correclty get time

The sticky bit and the interrupt pending bits can be used to ensure that
the correctu current time is read even if the interrupt to increment the
time has not been handled.

hifive: Correctly configure timer registers

The PWM device on the hifive will trigger interrupts if whenever the
compare registers are equal to the counter (even if the counter is not
enabled). As such, unused timer registers are set to the maximum value
and the inactive timer is set to 0 when not in use.

Additionally, the interrupt pending flags can be written to in order to
acknowledge the interrupts.

libplatsupport: Add hifive serial and pwm drivers

This implements the required drivers to pass sel4test