ANDROID: gki_defconfig: Enable MFD_SYSCON on x86

This option was accidentally skipped when it was added on arm64.

Bug: 144867487
Change-Id: Ifa87a894954ec9a26d5ab40e7d18e2f2f5e4f416
Signed-off-by: Alistair Delva <adelva@google.com>