Documentation: sysfs: Document Broadcom STB memc sysfs knobs

Document the "srpd" and "frequency" sysfs attributes exposed by
the brcmstb_memc driver.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220812222533.2428033-3-f.fainelli@gmail.com