[ARM] use asm/sections.h

Update to use the asm/sections.h header rather than declaring these
symbols ourselves.  Change __data_start to _data to conform with the
naming found within asm/sections.h.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/head-common.S

@@ -18,7 +18,7 @@
 __switch_data:
 	.long	__mmap_switched
 	.long	__data_loc			@ r4
-	.long	__data_start			@ r5
+	.long	_data				@ r5
 	.long	__bss_start			@ r6
 	.long	_end				@ r7
 	.long	processor_id			@ r4