arm: mxs: Enable booting of mx28 without battery

Section 4.1.2 of Freescale Application Note AN4199 describes the
configuration required to operate the mx28 from a 5V source without a
battery.

This patch changes the behaviour of the dropout control of the DC-DC
converter (refer to section 11.12.9 of the mx28 Application Processor
Reference Manual - Document Number: MCIMX28RM, Rev 2, 08/2013) to the
following:
 - Always use 4P2 Linear Regulator if CONFIG_SYS_MXS_VDD5V_ONLY is defined
 - Switch between 4P2 Linear Regulator and Battery, using whichever has
   the highest voltage if CONFIG_SYS_MXS_VDD5V_ONLY isnot set (this is
   the same as the pre-patch behaviour)

Signed-off-by: Graeme Russ <gruss@tss-engineering.com>
Signed-off-by: Damien Gotfroi <dgotfroi@greenwatch.be>

arch/arm/cpu/arm926ejs/mxs/spl_power_init.c

@@ -14,6 +14,13 @@
 
 #include "mxs_init.h"
 
+#ifdef CONFIG_SYS_MXS_VDD5V_ONLY
+#define DCDC4P2_DROPOUT_CONFIG	POWER_DCDC4P2_DROPOUT_CTRL_100MV | \
+				POWER_DCDC4P2_DROPOUT_CTRL_SRC_4P2
+#else
+#define DCDC4P2_DROPOUT_CONFIG	POWER_DCDC4P2_DROPOUT_CTRL_100MV | \
+				POWER_DCDC4P2_DROPOUT_CTRL_SRC_SEL
+#endif
 /**
  * mxs_power_clock2xtal() - Switch CPU core clock source to 24MHz XTAL
  *
@@ -303,8 +310,7 @@ static void mxs_power_init_4p2_params(void)
 
 	clrsetbits_le32(&power_regs->hw_power_dcdc4p2,
 		POWER_DCDC4P2_DROPOUT_CTRL_MASK,
-		POWER_DCDC4P2_DROPOUT_CTRL_100MV |
-		POWER_DCDC4P2_DROPOUT_CTRL_SRC_SEL);
+		DCDC4P2_DROPOUT_CONFIG);
 
 	clrsetbits_le32(&power_regs->hw_power_5vctrl,
 		POWER_5VCTRL_CHARGE_4P2_ILIMIT_MASK,