soc/ambiq/apollo5x: fix ARMV8_1_M_PMU_EVENTCNT being globally set

Other targets that define this symbol do it in the Kconfig file, and
gate it with a SOC_ model or series symbol. Defining a default in the
Kconfig.soc instead applies it on every build, which is not desired.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>