board/qemu/aarch64-virt/linux.config: add RTC PL031 driver

Qemu for the aarch64 virt emulate an RTC PL031 device.
Enable the kernel support to allow setting the system time.

"date" now return the current time:
Sun Jul  5 20:38:50 UTC 2020

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/qemu/aarch64-virt/linux.config

@@ -45,6 +45,7 @@ CONFIG_VIRTIO_CONSOLE=y
 CONFIG_HW_RANDOM=y
 CONFIG_HW_RANDOM_VIRTIO=y
 CONFIG_RTC_CLASS=y
+CONFIG_RTC_DRV_PL031=y
 CONFIG_VIRTIO_MMIO=y
 CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES=y
 CONFIG_MAILBOX=y