boards nrf54l15bsim: Do not delete grtc clock properties

Revert the change to this file done in
865b2456
In which these clock properties were added to the GRTC binding but
they were, at the same time, not added/removed for the simulated
target.
The author indicates this was done due to some test failing,
but at this point no test fails with these properties present also
in the simulated target, and there is no understanding of why they
would.
So let's avoid diverging the simulated target from the real
one.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>