x86: fsp: Only compile fsp_save_s3_stack if (SPL_)DM_RTC is enabled

This function calls rtc_write32, which has a different signature
depending on if (SPL_)DM_RTC is enabled or not. This could result in a
mismatch in SPL if DM_RTC was enabled but SPL_DM_RTC, as the non-DM
declaration would still be used in SPL even though the implementation
would be for non-DM_RTC. We are switching to the correct definitions in
the next commit, so this will become a compilation error. Since
fsp_save_s3_stack is not called from SPL, avoid compiling it if
(SPL_)DM_RTC is disabled.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>