irqchip/riscv-intc: Fix low-level interrupt handler setup for AIA

Use riscv_intc_aia_irq() as the low-level interrupt handler instead of the
existing riscv_intc_irq() default handler to make demultiplexing work
correctly.

Also print "using AIA" in the INTC boot banner when AIA is available.

Fixes: 3c46fc5b

 ("irqchip/riscv-intc: Add support for RISC-V AIA")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240226040746.1396416-2-apatel@ventanamicro.com