target/avr: Fix typo in gdb-xml feature name

While GDB doesn't provide an official XML for the gdbstub
protocol, it is still better to name it 'avr' instead of 'riscv'.

Fixes: 12b3540547 ("target/avr: CPU class: Add GDB support")
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20260219191955.83815-5-philmd@linaro.org>

gdb-xml/avr-cpu.xml

@@ -10,7 +10,7 @@
      register descriptions.  -->
 
 <!DOCTYPE feature SYSTEM "gdb-target.dtd">
-<feature name="org.gnu.gdb.riscv.cpu">
+<feature name="org.gnu.gdb.avr.cpu">
   <reg name="r0" bitsize="8" type="int" regnum="0"/>
   <reg name="r1" bitsize="8" type="int"/>
   <reg name="r2" bitsize="8" type="int"/>