[sw/silicon_creator] Preserve upper bits of the cpuctrl CSR
[sw/silicon_creator] Preserve upper bits of the cpuctrl CSR

This commit updates the way cpuctrl CSR is written in mask_rom_init()
such that its upper bits, more specifically `double_fault_seen` (bit 6)
and `double_fault_seen` (bit 7), are preserved.

Signed-off-by: Alphan Ulusoy <23b245cc5a07aacf75a9db847b24c67dee1707bf@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan