powerpc/dts: update MSI bindings doc for MPIC v4.3

Add compatible "fsl,mpic-msi-v4.3" for MPIC v4.3. MPIC v4.3 contains
MSIIR and MSIIR1. MSIIR supports 8 MSI registers and MSIIR1 supports
16 MSI registers, but uses different IBS and SRS shift. When using
MSIR1, the interrupt number is not consecutive. It is hard to use
'msi-available-ranges' to describe the ranges of the available
interrupt, so MPIC v4.3 does not support this property.

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
[scottwood@freescale.com: minor grammar fixes]
Signed-off-by: Scott Wood <scottwood@freescale.com>