powerpc/512x: dts: remove misplaced IRQ spec from 'soc' node (5125)

the 'soc' node in the MPC5125 "tower" board .dts has an '#interrupt-cells'
property although this node is not an interrupt controller

remove this erroneously placed property because starting with v3.13-rc1
lookup and resolution of 'interrupts' specs for peripherals gets misled
(tries to use the 'soc' as the interrupt parent which fails), emits
'no irq domain found' WARN() messages and breaks the boot process

[ best viewed with 'git diff -U5' to have DT node names in the context ]

Cc: Anatolij Gustschin <agust@denx.de>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>