dt-bindings: mmc: brcm,sdhci-brcmstb: correct number of reg entries

The binding should not allow infinite number of 'reg' entries, so add
strict limit.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220428081817.35382-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>