irqchip/riscv-imsic: Add kernel parameter to disable IPIs

When injecting IPIs to a set of harts, the IMSIC IPI support will do a
separate MMIO write to the SETIPNUM_LE register of each target hart. This
means on a platform where IMSIC is trap-n-emulated, there will be N MMIO
traps when injecting IPI to N target harts hence IMSIC IPIs will be slow on
such platforms compared to the SBI IPI extension.

Unfortunately, there is no DT, ACPI, or any other way of discovering
whether the underlying IMSIC is trap-n-emulated. Using MMIO write to the
SETIPNUM_LE register for injecting IPI is purely a software choice in the
IMSIC driver hence add a kernel parameter to allow users to disable IMSIC
IPIs on platforms with trap-n-emulated IMSIC.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250716123745.557585-1-apatel@ventanamicro.com

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