irqchip/riscv-imsic: Add device MSI domain support for platform devices

The Linux platform MSI support allows per-device MSI domains so add
a platform irqchip driver for RISC-V IMSIC which provides a base IRQ
domain with MSI parent support for platform device domains.

The IMSIC platform driver assumes that the IMSIC state is already
initialized by the IMSIC early driver.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20240307140307.646078-4-apatel@ventanamicro.com