[top_darjeeling] ROM patch driver does not enable fetch from SRAM

### desc

The Darjeeling [ROM patching driver](https://github.com/lowRISC/opentitan/blob/integrated_dev/sw/lib/sw/device/silicon_creator/rom_patch.c) loads patches in private SRAM but does not explicitly enable fetching from there.
When the SRAM controller `EXEC_EN` bit is not set prior to jumping to the patch, this prevents the patched second ROM to be executed.