Exynos542x: Make A7s boot with thumb-mode U-Boot on warm reset

On warm reset, all cores jump to the low_power_start function because iRAM
data is retained and because while executing iROM code all cores find
the jump flag 0x02020028 set. In low_power_start, cores check the reset
status and if true they clear the jump flag and jump back to 0x0.

The A7 cores do jump to 0x0 but consider following instructions as a Thumb
instructions which in turn makes them loop inside the iROM code instead of
jumping to power_down_core.

This issue is fixed by replacing the "mov pc" instruction with a "bx"
instruction which switches state along with the jump to make the execution
unit consider the branch target as an ARM instruction.

Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>

arch/arm/cpu/armv7/exynos/lowlevel_init.c

@@ -103,7 +103,7 @@ static void low_power_start(void)
 	reg_val = readl(EXYNOS5420_SPARE_BASE);
 	if (reg_val != CPU_RST_FLAG_VAL) {
 		writel(0x0, CONFIG_LOWPOWER_FLAG);
-		set_pc(0x0);
+		branch_bx(0x0);
 	}
 
 	reg_val = readl(CONFIG_PHY_IRAM_BASE + 0x4);