ARM: use r9 for gd

To be more EABI compliant and as a preparation for building
with clang, use the platform-specific r9 register for gd
instead of r8.

note: The FIQ is not updated since it is not used in u-boot,
and under discussion for the time being.

The following checkpatch warning is ignored:
WARNING: Use of volatile is usually wrong: see
Documentation/volatile-considered-harmful.txt

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
diff --git a/arch/arm/cpu/armv7/lowlevel_init.S b/arch/arm/cpu/armv7/lowlevel_init.S
index 82b2b86..69e3053 100644
--- a/arch/arm/cpu/armv7/lowlevel_init.S
+++ b/arch/arm/cpu/armv7/lowlevel_init.S
@@ -22,11 +22,11 @@
 	ldr	sp, =CONFIG_SYS_INIT_SP_ADDR
 	bic	sp, sp, #7 /* 8-byte alignment for ABI compliance */
 #ifdef CONFIG_SPL_BUILD
-	ldr	r8, =gdata
+	ldr	r9, =gdata
 #else
 	sub	sp, #GD_SIZE
 	bic	sp, sp, #7
-	mov	r8, sp
+	mov	r9, sp
 #endif
 	/*
 	 * Save the old lr(passed in ip) and the current lr to stack