dt-bindings: arm: stm32: add simple-mfd compatible for tamp node

The stm32mp1 TAMP (Tamper and backup registers) does tamper detection
and features 32 backup registers that, being in the RTC domain, may
survive even with Vdd switched off.

This makes it suitable for use to communicate a reboot mode from OS
to bootloader via the syscon-reboot-mode binding. Add a "simple-mfd"
to support probing such a child node. The actual reboot mode
node could then be defined in a board.dts or fixed up by the bootloader.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>