ARM: add documentation for finding start of physical memory

Occasionally, there's a question about the method we use to find the
start of physical memory.  Add some documentation so we don't have to
keep repeating outselves on the mailing list.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/boot/compressed/head.S

@@ -168,9 +168,26 @@ not_angel:
 		.text
 
 #ifdef CONFIG_AUTO_ZRELADDR
-		@ determine final kernel image address
+		/*
+		 * Find the start of physical memory.  As we are executing
+		 * without the MMU on, we are in the physical address space.
+		 * We just need to get rid of any offset by aligning the
+		 * address.
+		 *
+		 * This alignment is a balance between the requirements of
+		 * different platforms - we have chosen 128MB to allow
+		 * platforms which align the start of their physical memory
+		 * to 128MB to use this feature, while allowing the zImage
+		 * to be placed within the first 128MB of memory on other
+		 * platforms.  Increasing the alignment means we place
+		 * stricter alignment requirements on the start of physical
+		 * memory, but relaxing it means that we break people who
+		 * are already placing their zImage in (eg) the top 64MB
+		 * of this range.
+		 */
 		mov	r4, pc
 		and	r4, r4, #0xf8000000
+		/* Determine final kernel image address. */
 		add	r4, r4, #TEXT_OFFSET
 #else
 		ldr	r4, =zreladdr