ARM: shmobile: r8a7791: Correct mask for GIC PPI interrupts

R-Car M2-W (r8a7791) contains two Cortex-A15 cores, hence the second
interrupt specifier cell for Private Peripheral Interrupts should use
"GIC_CPU_MASK_SIMPLE(2)".

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