MIPS: EMMA2RH: Remove EMMA2RH_CPU_CASCADE

Although all EMMAxxx SoCs can support IP2 and IP3 hardware interrupts,
current EMMA2RH plat_irq_dispatch() supports IP2 only.  We can make it
configurable in the future, but for the time being, would like to make
things explicitly allcated to IP2 in accordance with plat_irq_dispatch().

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1388/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>