xtensa: make PLATFORM_DEFAULT_MEM parameters configurable

This allows using xtfpga platform with both MMU and noMMU cores.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/platforms/xtfpga/include/platform/hardware.h

@@ -17,8 +17,8 @@
 
 /* Memory configuration. */
 
-#define PLATFORM_DEFAULT_MEM_START 0x00000000
-#define PLATFORM_DEFAULT_MEM_SIZE  0x04000000
+#define PLATFORM_DEFAULT_MEM_START CONFIG_DEFAULT_MEM_START
+#define PLATFORM_DEFAULT_MEM_SIZE  CONFIG_DEFAULT_MEM_SIZE
 
 /* Interrupt configuration. */