Skip clearing SRAM in ROM by default.

[zhalvorsen]

CaliptraIntegrationSpecification.md says that the SoC should zeroize the SRAM before pulling Caliptra out of reset. There is assembly code in the ROM to also clear SRAM on a cold reboot. In simulation clearing SRAM can take a long time. I'm hoping to conditionally remove clearing the SRAM in ROM. I created a draft PR to pull it behind a feature flag (#1622). I wanted to get confirmation from multiple leads before putting it in for review.