configure: Fixes for --enable-xxx

When the --enable-psci=no or --enable-gicv3=no form of configure option
was passed our configure script was setting the option to yes.  This
changes the logic slightly to fix the problem.

Signed-off-by: Geoff Levand <geoff@infradead.org>
[Mark: fit with exiting USE_FEATURE naming scheme, fix whitespace]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>