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>
diff --git a/board/sunxi/board.c b/board/sunxi/board.c
index d1d7f9f..e075c13 100644
--- a/board/sunxi/board.c
+++ b/board/sunxi/board.c
@@ -820,6 +820,7 @@
 #endif
 	};
 
+#ifdef CONFIG_PINE64_DT_SELECTION
 /* Differentiate the two Pine64 board DTs by their DRAM size. */
 	if (strstr(name, "-pine64") && strstr(cmp_str, "-pine64")) {
 		if ((gd->ram_size > 512 * 1024 * 1024))
@@ -829,5 +830,7 @@
 	} else {
 		return strcmp(name, cmp_str);
 	}
+#endif
+	return strcmp(name, cmp_str);
 }
 #endif