ARM: dts: exynos: align PPMU event node names with dtschema

Use hyphen instead of underscore and align the PPMU event node name with
dtschema.  The event-name property must match the node name, by the
design of devfreq events and PPMU driver.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20210920071753.38560-3-krzysztof.kozlowski@canonical.com