dt-bindings: mailbox: mpfs: fix reg properties

When the binding for this was originally written, and later modified,
mistakes were made - and the precise nature of the later modification
should have been a giveaway, but alas I was naive at the time.

A more correct modelling of the hardware is to use two syscons and have
a single reg entry for the mailbox, containing the mailbox region. The
two syscons contain the general control/status registers for the mailbox
and the interrupt related registers respectively. The reason for two
syscons is that the same mailbox is present on the non-SoC version of
the FPGA, which has no interrupt controller, and the shared part of the
rtl was unchanged between devices.

This is now coming to a head, because the control/status registers share
a register region with the "tvs" (temperature & voltage sensors)
registers and, as it turns out, people do want to monitor temperatures
and voltages...

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>