dts: bindings: riscv: add APLIC binding

Add devicetree binding for the RISC-V Advanced Platform-Level
Interrupt Controller (APLIC). Uses a single compatible "riscv,aplic"
with an optional msi-parent property: when present the driver operates
in MSI delivery mode, when absent direct delivery mode is assumed.

Signed-off-by: Afonso Oliveira <afonso.oliveira707@gmail.com>