arm64: Add missing Kconfig for CONFIG_STRICT_DEVMEM

The Kconfig for CONFIG_STRICT_DEVMEM is missing despite being
used in mmap.c. Add it.
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig.debug

@@ -6,6 +6,20 @@ config FRAME_POINTER
 	bool
 	default y
 
+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.
+
 config EARLY_PRINTK
 	bool "Early printk support"
 	default y