sunxi: disable Pine A64 model detection code on other boards

The Pine A64 Plus/non-Plus model detection code is now built on all
64-bit ARM SoCs, even if the code cannot be triggered when H5/H6 is in
use.

Disable them when the board is Pine A64 by adding a Kconfig option that
is only selected on Pine A64.

On GCC 7.3.1 this makes the size of the function reduces 184 bytes, and
saves a 104 byte strstr() function, then makes SPL on H6 succeed to
build.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>