ARM: dts: stm32: Add arm,no-tick-in-suspend to STM32MP15xx STGEN timer

STM32MP15xx RM0436 Rev 6 section 46.3 System timer generator (STGEN) states
"
Arm recommends that the system counter is in an always-on power domain.
This is not supported in the current implementation, therefore STGEN should
be saved and restored before Standby mode entry, and restored at Standby
exit by secure software.
...
"
Instead of piling up workarounds in the firmware which is difficult to
update, add "arm,no-tick-in-suspend" DT property into the timer node to
indicate the timer is stopped in suspend, and let the kernel fix the
timer up.

Fixes: 8471a202 ("ARM: dts: stm32: add stm32mp157c initial support")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>