fix: update arm64 kernel config with proper source

This config was created by starting with amd64 config we have, going
through `make olddefconfig`, and then adding a few options required at
least for QEMU emulation:

* `AMBA` serial console (`ttyAMA0`)
* RTC clock, `rtc_efi`

This kernel boots fine under QEMU.

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>