MAINTAINERS: Merge ARM/Renesas ARM64 and ARM/SH-Mobile ARM architectures

There is not much point in having separate entries for the ARM/Renesas
ARM64 and ARM/SH-Mobile ARM architectures, as they have the same
maintainers, use the same development collaboration infrastructure, and
share many files.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/a869b8afdc47aa637ebeefcc1ca7bc61244f34b9.1660898008.git.geert+renesas@glider.be