MAINTAINERS: Add entry for RISC-V AIA drivers

Add myself as maintainer for RISC-V AIA drivers including the
RISC-V INTC driver which supports both AIA and non-AIA platforms.

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/20240307140307.646078-10-apatel@ventanamicro.com