arm64: defconfig: enable RTC and audio support on Kontron sl28 boards

Although the Kontron SMARC-sAL28 board is now supported, there is still
missing driver support for the hardware clock, audio interface and audio
codec. Let's enable them as modules (where possible).

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>