ARM: implement CONFIG_STRICT_DEVMEM by disabling access to RAM via /dev/mem

There are very few legitimate use cases, if any, for directly accessing
system RAM through /dev/mem.  So let's mimic what they do on x86 and
forbid it when CONFIG_STRICT_DEVMEM is turned on.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>

arch/arm/Kconfig.debug

@@ -2,6 +2,20 @@ menu "Kernel hacking"
 
 source "lib/Kconfig.debug"
 
+config STRICT_DEVMEM
+	bool "Filter access to /dev/mem"
+	depends on MMU
+	---help---
+	  If this option is disabled, you allow userspace (root) access to all
+	  of memory, including kernel and userspace memory. Accidental
+	  access to this is obviously disastrous, but specific access can
+	  be used by people debugging the kernel.
+
+	  If this option is switched on, the /dev/mem file only allows
+	  userspace access to memory mapped peripherals.
+
+          If in doubt, say Y.
+
 # RMK wants arm kernels compiled with frame pointers or stack unwinding.
 # If you know what you are doing and are willing to live without stack
 # traces, you can get a slightly smaller kernel by setting this option to