vexpress64: move hardware setting from defconfig to Kconfig

vexpress64: move hardware setting from defconfig to Kconfig

The defconfigs for the Arm Juno board and the FVP model are quite large,
setting a lot of platform-fixed variables like SYS_TEXT_BASE.
As those values are not really a user choice, let's provide default
values for them in our Kconfig file, so a lot of cruft can be removed
from the defconfig files.
This also moves the driver selection out of there, since this is again
not something a user should really decide on. Instead we allow users to
enable or disable subsystems, and select the appropriate drivers based
on that in the Kconfig file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>