rockpro64 timer: Allocate IRQ from FDT directly

This traverses the FDT directly when initialising the driver for the
rockpro64 timer so that the IRQ for the second timer can be derived from
the IRQ allocated to the first device.

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>

libplatsupport: refactor more timers

- The following platforms are changed: am335x,
fvp, hikey, rockpro64
- All platforms pass build
- Except for fvp which doesn't have a machine handy,
the other 3 platforms pass sel4test
- For each platform, the ltimer.c and device-specific
code is more intertwined than I would have liked,
including things like device code branching on
`LTIMER` constants, but I think cleaning the
interface is a project for later

Added rockpro64 Platform support

This adds enough to use timer and serial for sel4test