drivers: timer: riscv_machine_timer: obtain registers/IRQ from DT

Obtain machine timer addresses and IRQ from Devicetree. Note that driver
supports multiple compatibles because mtime/mtimecmp registers are
implemented in different ways depending on the vendor. That means
Devicetree representations can be slightly different and so code to
collect the information needs to treat each compatible differently.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>