arm: use canonical sub mnemonic

Building some arm boards with older binutils may produce errors like this:

---8<---
crt0.S: Assembler messages:
crt0.S:70: Error: register expected, not '#(184)' -- `sub sp,#(184)'
--->8---

Use canonical version of the subtract mnemonic to avoid those issues.

Reported-by: Alexey Smishlayev <alexey@xtech2.lv>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>

arch/arm/cpu/armv7/lowlevel_init.S

@@ -24,7 +24,7 @@ ENTRY(lowlevel_init)
 #ifdef CONFIG_SPL_BUILD
 	ldr	r9, =gdata
 #else
-	sub	sp, #GD_SIZE
+	sub	sp, sp, #GD_SIZE
 	bic	sp, sp, #7
 	mov	r9, sp
 #endif

arch/arm/lib/crt0.S

@@ -67,7 +67,7 @@ ENTRY(_main)
 	ldr	sp, =(CONFIG_SYS_INIT_SP_ADDR)
 #endif
 	bic	sp, sp, #7	/* 8-byte alignment for ABI compliance */
-	sub	sp, #GD_SIZE	/* allocate one GD above SP */
+	sub	sp, sp, #GD_SIZE	/* allocate one GD above SP */
 	bic	sp, sp, #7	/* 8-byte alignment for ABI compliance */
 	mov	r9, sp		/* GD is above SP */
 	mov	r0, #0