maintainers: riscv: Add myself as collaborator
maintainers: riscv: Add myself as collaborator

Add myself as collaborator to cake a closer look at the patches getting
in.

Signed-off-by: Carlo Caione <ca4506c8c654a4ccf75802e86b5d162c1e96f065@baylibre.com>

galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr