add memory regions to soc.svd

svd2rust does not recognize memory regions, but we'd like to
make an access crate for Rust that does.

This patch adds memory regions to soc.svd using the "vendorExtensions"
tag, as specified in https://www.keil.com/pack/doc/cmsis/SVD/html/svd_Format_pg.html

The vendorExtensions is added as a block after the Peripherals level, and has a format
like this:

```xml
        <memoryRegion>
            <name>SRAM</name>
            <baseAddress>0x10000000</baseAddress>
            <size>0x00020000</size>
        </memoryRegion>
        <memoryRegion>
            <name>VEXRISCV_DEBUG</name>
            <baseAddress>0xEFFF0000</baseAddress>
            <size>0x00000100</size>
        </memoryRegion>
        <memoryRegion>
            <name>CSR</name>
            <baseAddress>0xF0000000</baseAddress>
            <size>0x00040000</size>
        </memoryRegion>
```

litex/soc/integration/export.py

@@ -428,6 +428,15 @@ def get_csr_svd(soc, vendor="litex", name="soc", description=None):
             svd.append('            </interrupt>')
         svd.append('        </peripheral>')
     svd.append('    </peripherals>')
+    if len(soc.mem_regions) > 0:
+        svd.append('    <vendorExtensions>')
+        for name, region in soc.mem_regions.items():
+            svd.append('        <memoryRegion>')
+            svd.append('            <name>{}</name>'.format(name.upper()))
+            svd.append('            <baseAddress>0x{:08X}</baseAddress>'.format(region.origin))
+            svd.append('            <size>0x{:08X}</size>'.format(region.size))
+            svd.append('        </memoryRegion>')
+        svd.append('    </vendorExtensions>')
     svd.append('</device>')
     return "\n".join(svd)