MIPS: u-boot.lds: merge all BSS sections and introduce symbols __bss_[start|end]

These symbols are used in later patches for as addresses for
clearing the BSS area in the relocated U-Boot image.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/arch/mips/cpu/u-boot.lds b/arch/mips/cpu/u-boot.lds
index 58a49b2..6980b86 100644
--- a/arch/mips/cpu/u-boot.lds
+++ b/arch/mips/cpu/u-boot.lds
@@ -70,13 +70,14 @@
 	uboot_end_data = .;
 
 	. = ALIGN(4);
-	.sbss : {
-		*(.sbss*)
-	}
-
 	.bss : {
-		*(.bss*)
+		__bss_start = .;
+		*(.sbss.*)
+		*(.bss.*)
+		*(COMMON)
 		. = ALIGN(4);
+		__bss_end = .;
 	}
+
 	uboot_end = .;
 }