ARM: shmobile: defconfig: Disable staging

After the DT conversion of SH-Mobile and Armadillo-800-EVA display
support, and the removal of the EMMA Mobile USB Gadget staging driver,
there is no longer a need to enable support for board staging or any
other staging drivers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/7f1fa862cb508a5d29947380b72324be027ac76f.1705933596.git.geert+renesas@glider.be