drivers: counter: stm32_stgen: add STGEN driver

STGEN is the platform timer. It generates a time-count value that provides
a consistent view of time for multiple processors and other blocks in a
dev

drivers: counter: stm32_stgen: add STGEN driver

STGEN is the platform timer. It generates a time-count value that provides
a consistent view of time for multiple processors and other blocks in a
device. It is physically linked to the ARM generic timer.

Add the STGEN driver that is in charge of configuring the ARM generic
timer source and send an SMC to the BL31 monitor to update the CP15
register. This driver is only compatible for 64bits platforms.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>

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