cache: sifive_ccache: Partially convert to a platform driver

Commit 8ec99b03 ("irqchip/sifive-plic: Convert PLIC driver into a
platform driver") broke ccache initialization because the PLIC IRQ
domain is no longer available during an arch_initcall:

  [    0.087229] irq: no irq domain found for interrupt-controller@c000000 !
  [    0.087255] CCACHE: Could not request IRQ 0

Fix this by moving the IRQ handling code to a platform driver.

Fixes: 8ec99b03 ("irqchip/sifive-plic: Convert PLIC driver into a platform driver")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>