riscv: dts: microchip: fix mailbox description

riscv: dts: microchip: fix mailbox description

When the binding for the mailbox on PolarFire SoC 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.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>