vexpress64: Kconfig: tidy up

The FVP and Juno settings were identical, but duplicated, so I removed
the duplication with this patch.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
[trini: Adjust logic to keep if/endif in the file]
Signed-off-by: Tom Rini <trini@konsulko.com>