mx5: Mark lowlevel_init board-specific code

The mx5 lowlevel_init.S contains board-specific code based on the reference
design. Let's keep it since it avoids creating new lowlevel_init files and it
may be used by many boards. But add a config to make it optional in order not to
cause issues on boards not following this part of the reference design.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Matt Sealey <matt@genesi-usa.com>
Acked-by: Stefano Babic <sbabic@denx.de>

arch/arm/cpu/armv7/mx5/lowlevel_init.S

@@ -396,7 +396,7 @@ ENTRY(lowlevel_init)
 	mov r10, lr
 	mov r4, #0	/* Fix R4 to 0 */
 
-#if defined(CONFIG_MX51)
+#if defined(CONFIG_SYS_MAIN_PWR_ON)
 	ldr r0, =GPIO1_BASE_ADDR
 	ldr r1, [r0, #0x0]
 	orr r1, r1, #1 << 23

doc/README.imx5

@@ -15,3 +15,8 @@ i.MX5x SoCs.
     mode), which causes the effect of this failure to be much lower (in terms
     of frequency deviation), avoiding system failure, or at least decreasing
     the likelihood of system failure.
+
+1.2 CONFIG_SYS_MAIN_PWR_ON: Trigger MAIN_PWR_ON upon startup.
+    This option should be enabled for boards having a SYS_ON_OFF_CTL signal
+    connected to GPIO1[23] and triggering the MAIN_PWR_ON signal like in the
+    reference designs.

include/configs/mx51_efikamx.h

@@ -261,5 +261,6 @@
 
 #define CONFIG_SYS_DDR_CLKSEL		0
 #define CONFIG_SYS_CLKTL_CBCDR		0x59E35145
+#define CONFIG_SYS_MAIN_PWR_ON
 
 #endif

include/configs/mx51evk.h

@@ -235,6 +235,7 @@
 
 #define CONFIG_SYS_DDR_CLKSEL	0
 #define CONFIG_SYS_CLKTL_CBCDR	0x59E35100
+#define CONFIG_SYS_MAIN_PWR_ON
 
 /*-----------------------------------------------------------------------
  * FLASH and environment organization

include/configs/vision2.h

@@ -196,6 +196,7 @@
 /* 166 MHz DDR RAM */
 #define CONFIG_SYS_DDR_CLKSEL		0
 #define CONFIG_SYS_CLKTL_CBCDR		0x19239100
+#define CONFIG_SYS_MAIN_PWR_ON
 
 #define CONFIG_SYS_NO_FLASH