MIPS: Move CONFIG_SYS_TEXT_BASE to Kconfig

Move CONFIG_SYS_TEXT_BASE to Kconfig, and add default values in board
Kconfig files matching what was present in their config headers. This
will make it cleaner to conditionalise the value for Malta based on 32
vs 64 bit builds.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
diff --git a/board/qca/ap121/Kconfig b/board/qca/ap121/Kconfig
index f7e768a..c3ecc8f 100644
--- a/board/qca/ap121/Kconfig
+++ b/board/qca/ap121/Kconfig
@@ -9,4 +9,7 @@
 config SYS_CONFIG_NAME
 	default "ap121"
 
+config SYS_TEXT_BASE
+	default 0x9f000000
+
 endif