arm: socfpga: make SPL_TEXT_BASE overridable

To boot from fpga on socfpga gen5, we need to set CONFIG_SPL_TEXT_BASE to
0xC0000000 (hps2fpgaslaves base address).

Since converting CONFIG_SPL_TEXT_BASE to Kconfig hasn't been successful so
far, let's make this value overridable in socfpga_common.h, so that we can
have different board configs override this in socfpga_common.h.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
diff --git a/include/configs/socfpga_common.h b/include/configs/socfpga_common.h
index 181af9b..191204b 100644
--- a/include/configs/socfpga_common.h
+++ b/include/configs/socfpga_common.h
@@ -248,8 +248,10 @@
  * 0xFFEz_zzzz ...... Malloc area (grows up to top)
  * 0xFFE3_FFFF ...... End of SRAM (top)
  */
+#ifndef CONFIG_SPL_TEXT_BASE
 #define CONFIG_SPL_TEXT_BASE		CONFIG_SYS_INIT_RAM_ADDR
 #define CONFIG_SPL_MAX_SIZE		CONFIG_SYS_INIT_RAM_SIZE
+#endif
 
 #if defined(CONFIG_TARGET_SOCFPGA_ARRIA10)
 /* SPL memory allocation configuration, this is for FAT implementation */