ARM: ep93xx: Select GENERIC_IRQ_MULTI_HANDLER directly

ep93xx currently relies of CONFIG_ARM_VIC to select
GENERIC_IRQ_MULTI_HANDLER. Given that this is logically a platform
architecture property, add the selection of GENERIC_IRQ_MULTI_HANDLER
at the platform level.

Further patches will remove the selection from the irqchip side.

Reported-by: Marc Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Hector Martin <marcan@marcan.st>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>