ARM: dts: keystone: k2g: Rename message-manager node

Rename message-manager instance node name to be better aligned with
current style of device tree nodes for mailboxes.

Reviewed-by: Tero Kristo <kristo@kernel.org>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>