irqchip/sifive-plic: remove user selectability of SIFIVE_PLIC

irqchip/sifive-plic: remove user selectability of SIFIVE_PLIC

The SiFive PLIC driver is used by all current implementations, including
those that do not have a SiFive PLIC. The current driver supports more
than just SiFive PLICs at present and, where possible, future PLIC
implementations will also use this driver. As every supported RISC-V SoC
selects the driver directly in Kconfig.socs there's no point in exposing
this kconfig option to users.

The Kconfig help text, in its current form, is misleading. There's no
point doing anything about that though, as it will no longer be user
selectable. Remove it.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221118104300.85016-2-conor@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>