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

Documentation/ABI/testing/sysfs-platform-brcmstb-memc

+What:		/sys/bus/platform/devices/*/srpd
+Date:		July 2022
+KernelVersion:	5.21
+Contact:	Florian Fainelli <f.fainelli@gmail.com>
+Description:
+		Self Refresh Power Down (SRPD) inactivity timeout counted in
+		internal DDR controller clock cycles. Possible values range
+		from 0 (disable inactivity timeout) to 65535 (0xffff).
+
+What:		/sys/bus/platform/devices/*/frequency
+Date:		July 2022
+KernelVersion:	5.21
+Contact:	Florian Fainelli <f.fainelli@gmail.com>
+Description:
+		DDR PHY frequency in Hz.