test/py: Update u_boot_utils.find_ram_base to bypass the low 2MiB memory

On some RISC-V targets the low memory is protected that prevents
S-mode U-Boot from access.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/test/py/u_boot_utils.py b/test/py/u_boot_utils.py
index bf2a0fc..939d82e 100644
--- a/test/py/u_boot_utils.py
+++ b/test/py/u_boot_utils.py
@@ -237,10 +237,11 @@
             raise Exception('Failed to find RAM bank start in `bdinfo`')
 
     # We don't want ram_base to be zero as some functions test if the given
-    # address is NULL (0). Let's add 2MiB then (size of an ARM LPAE/v8 section).
+    # address is NULL (0). Besides, on some RISC-V targets the low memory
+    # is protected that prevents S-mode U-Boot from access.
+    # Let's add 2MiB then (size of an ARM LPAE/v8 section).
 
-    if ram_base == 0:
-        ram_base += 1024 * 1024 * 2
+    ram_base += 1024 * 1024 * 2
 
     return ram_base