docs(maintainers): add RD-V3 variants to maintained paths

add the path for RD-V3 and variants to the maintained paths under
Neoverse reference design platforms.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: Ib9323143bedf49304ed6f07b20dc39b8821464c5
1 file changed