irqchip/pruss: Simplify the TI_PRUSS_INTC Kconfig

The TI PRUSS INTC irqchip driver handles the local interrupt controller
which is a child device of it's parent PRUSS/ICSSG device. The driver
was upstreamed in parallel with the PRUSS platform driver, and was
configurable independently previously. The PRUSS interrupt controller
is an integral part of the overall PRUSS software architecture, and is
not useful at all by itself.

Simplify the TI_PRUSS_INTC Kconfig dependencies by making it silent and
selected automatically when the TI_PRUSS platform driver is enabled.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: David Lechner <david@lechnology.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210108162901.6003-1-s-anna@ti.com