MIPS: start.S: simplify relocation offset calculation

The current code uses four instructions and a
temporary register to calculate the relocation
offset and to adjust the gp register.

The relocation offset can be calculated directly
from the CONFIG_SYS_MONITOR_BASE constant and from
the destination address. The resulting offset can
be used to adjust the gp pointer.

This approach makes the code a bit simpler because
it needs two instructions only.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>

arch/mips/cpu/mips32/start.S

@@ -283,19 +283,13 @@ relocate_code:
 	move	s2, a2			# save destination address in s2
 
 	li	t0, CONFIG_SYS_MONITOR_BASE
+	sub	s1, s2, t0		# s1 <-- relocation offset
+
 	la	t3, in_ram
 	lw	t2, -12(t3)		# t2 <-- uboot_end_data
 	move	t1, a2
 
-	/*
-	 * Fix $gp:
-	 *
-	 * New $gp = (Old $gp - CONFIG_SYS_MONITOR_BASE) + Destination Address
-	 */
-	move	t6, gp
-	sub	gp, CONFIG_SYS_MONITOR_BASE
-	add	gp, a2			# gp now adjusted
-	sub	s1, gp, t6		# s1 <-- relocation offset
+	add	gp, s1			# adjust gp
 
 	/*
 	 * t0 = source address

arch/mips/cpu/mips64/start.S

@@ -162,19 +162,13 @@ relocate_code:
 	move	s2, a2			# save destination address in s2
 
 	dli	t0, CONFIG_SYS_MONITOR_BASE
+	dsub	s1, s2, t0		# s1 <-- relocation offset
+
 	dla	t3, in_ram
 	ld	t2, -24(t3)		# t2 <-- uboot_end_data
 	move	t1, a2
 
-	/*
-	 * Fix $gp:
-	 *
-	 * New $gp = (Old $gp - CONFIG_SYS_MONITOR_BASE) + Destination Address
-	 */
-	move	t8, gp
-	dsub	gp, CONFIG_SYS_MONITOR_BASE
-	dadd	gp, a2			# gp now adjusted
-	dsub	s1, gp, t8		# s1 <-- relocation offset
+	dadd	gp, s1			# adjust gp
 
 	/*
 	 * t0 = source address