dt-bindings: clock: mediatek: convert PCIESYS to the json-schema clock

dt-bindings: clock: mediatek: convert PCIESYS to the json-schema clock

This helps validating DTS files. Introduced changes:
1. Documented "reg" property
2. Dropped "syscon" as it was incorrectly used
3. Adjusted nodename, "compatible" and "reg" in example

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240214061233.24645-3-zajec5@gmail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>