Remove the following warnings seen when building with W=1.
Warning (unique_unit_address): /soc/timer@40000c00: duplicate unit-address
(also used in node /soc/timers@40000c00)
This approach is based on some discussions[1], to restructure the dtsi
and dts files.

Timer5 is enabled by default on stm32f7 series, to act as clockevent. In
order to get rid of the W=1 warning, and be compliant with dt-schemas
(e.g. dtbs_check):
- In stm32f746.dtsi:
  . Keep the more complete timers5 description
  . Remove the most simple timer5 node that is duplicate
- In each board:
  . adopt "st,stm32-timer" compatible for timers5, also add the interrupt
  . use /delete-property/ and /delete-node/ so the it matches the
    clockevent bindings

Note: all this is done in one shot (e.g. not split) to keep clockevent
functionality.

[1] https://lore.kernel.org/linux-arm-kernel/Yaf4jiZIp8+ndaXs@robh.at.kernel.org/Signed-off-by: NFabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: NAlexandre Torgue <alexandre.torgue@foss.st.com>