drivers: systick: implement option for sys_clock_cycle_get_64()

This driver, due to its limited 24-bits counter, is already tracking a
cycle count in software. Allow that count to be a 64-bits value so this
won't wrap in a matter of only a few seconds when the hardware clock
is fast.

This is very cheap to do as expensive math operations (i.e. divisions)
are performed only on counter intervals whose values fit in 32 bits like
before.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>