ARM: dts: r8a73a4: Correct mask for GIC PPI interrupts

R-Mobile APE6 (r8a73a4) contains four Cortex-A15 and four Cortex-A7
cores, hence the second interrupt specifier cell for Private Peripheral
Interrupts should use "GIC_CPU_MASK_SIMPLE(8)", so GIC interrupts are
delivered to all 8 processor cores.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>