boards: arm: Add RTC clock source for mikroe_mini_m4_for_stm32

The `mikroe_mini_m4_for_stm32` board doesn't have its RTC node
enabled, and is failing the following test:

`tests/benchmarks/footprints/benchmark.kernel.footprints.pm`

This board seems to have been missed out from 44b8370, let's
enable the rtc & clk_lsi here.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
(cherry picked from commit 54573e7c)