irqchip/loongson-liointc: Use architecture register to get coreid

fa84f893 ("irqchip/loongson-liointc: Fix build error for
LoongArch") replaced get_ebase_cpunum with physical processor
id from SMP facilities. However that breaks MIPS non-SMP build
and makes booting from other cores inpossible on non-SMP kernel.

Thus we revert get_ebase_cpunum back and use get_csr_cpuid for
LoongArch.

Fixes: fa84f893 ("irqchip/loongson-liointc: Fix build error for LoongArch")
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220609175242.977-1-jiaxun.yang@flygoat.com