powerpc/fsl_msi: support vmpic msi with mpic 4.3

The new MSI block in MPIC 4.3 added the MSIIR1 register,
with a different layout, in order to support 16 MSIR
registers. The msi binding was also updated so that
the "reg" reflects the newly introduced MSIIR1 register.
Virtual machines advertise these msi nodes by using the
compatible "fsl,vmpic-msi-v4.3" so add support for it.

Signed-off-by: Laurentiu Tudor <Laurentiu.Tudor@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>