irqchip/ls-scfg-msi: Add LS1046a MSI support

LS1046a includes 4 MSIRs, each MSIR is assigned a dedicate GIC
SPI interrupt and provides 32 MSI interrupts. Compared to previous
MSI, LS1046a's IBS(interrupt bit select) shift is changed to 2 and
total MSI interrupt number is changed to 128.

The patch adds structure 'ls_scfg_msir' to describe MSIR setting and
'ibs_shift' to store the different value between the SoCs.

Signed-off-by: Minghuan Lian <Minghuan.Lian@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>