MAINTAINERS: Add myself as a riscv reviewer

The goal is for me to get a kernel.org account and then send pull requests
in order to relieve some pressure from Palmer and make our workflow
smoother.

Signed-off-by: Alexandre Ghiti <[email protected]>
Enthusiastically-Supported-by: Björn Töpel <[email protected]>
Acked-by: Conor Dooley <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: Alexandre Ghiti

MAINTAINERS

@@ -20280,6 +20280,7 @@ RISC-V ARCHITECTURE
 M:	Paul Walmsley <[email protected]>
 M:	Palmer Dabbelt <[email protected]>
 M:	Albert Ou <[email protected]>
+R:	Alexandre Ghiti <[email protected]>
 L:	[email protected]
 S:	Supported
 Q:	https://patchwork.kernel.org/project/linux-riscv/list/