memory: Describe the MEMORY Kconfig entry

Write short description about the entire memory controllers section.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

drivers/memory/Kconfig

@@ -5,6 +5,12 @@
 
 menuconfig MEMORY
 	bool "Memory Controller drivers"
+	help
+	  This option allows to enable specific memory controller drivers,
+	  useful mostly on embedded systems.  These could be controllers
+	  for DRAM (SDR, DDR), ROM, SRAM and others.  The drivers features
+	  vary from memory tuning and frequency scaling to enabling
+	  access to attached peripherals through memory bus.
 
 if MEMORY