modules: nordic: nrfx: workaround MDK erratas symbol for nRF54L05 & L10

MDK 8.68.1 uses nRF54L15_XXAA symbol for determining whether
given errata is applicable or not for nRF54L Series SoCs.
This causes all erratas to be disabled for nRF54L05 and nRF54L10.
To mitigate this DEVELOP_IN_NRF54L15 symbol needs to be defined,
and removed once new MDK is integrated.

Signed-off-by: Nikodem Kastelik <nikodem.kastelik@nordicsemi.no>