boards: arm64: doc: only list index files
boards: arm64: doc: only list index files

So that rst files located in a certain board are not listed in board
level table of contents.

Signed-off-by: Qi Feng <4f55eadd444e6c5672e2c0fc69a46f63b0ba7c30@arm.com>

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