arm: imx8m: fix build error when PHYS_SDRAM_2 not defined

- If total memory amount is 3 GiB or less, PHYS_SDRAM_2 memory
  range is not used. Enable setup of memory range only if
  PHYS_SDRAM_2 is defined.
Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>

arch/arm/mach-imx/imx8m/soc.c

@@ -266,7 +266,9 @@ int dram_init_banksize(void)
 			puts("CONFIG_NR_DRAM_BANKS is too small for SDRAM_2\n");
 			return -1;
 		}
+#ifdef PHYS_SDRAM_2
 		gd->bd->bi_dram[bank].start = PHYS_SDRAM_2;
+#endif
 #ifdef PHYS_SDRAM_2_SIZE
 		gd->bd->bi_dram[bank].size = PHYS_SDRAM_2_SIZE;
 #else