ARM: shmobile: Add missing dts files to MAINTAINERS

Add some more patterns to the ARM/SHMOBILE ARM ARCHITECTURE
section of MAINTAINERS to complete the list of maintained dts files.

As of writing dts files covered are:
* arch/arm/boot/dts/gr-peach-audiocamerashield.dtsi
* arch/arm/boot/dts/iwg20d-q7-common.dtsi
* arch/arm/boot/dts/iwg20d-q7-dbcm-ca.dtsi

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>