test: dm: Add test cases for FWU Metadata uclass

Add test cases for accessing the FWU Metadata on the sandbox
platform. The sandbox platform also uses the metadata access driver
for GPT partitioned block devices.

The FWU feature will be tested on the sandbox64 variant with a raw
capsule. Remove the FIT capsule testing from sandbox64 defconfig --
the FIT capsule test will be run on the sandbox_flattree variant.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/arch/sandbox/Kconfig b/arch/sandbox/Kconfig
index 35508c6..96b3402 100644
--- a/arch/sandbox/Kconfig
+++ b/arch/sandbox/Kconfig
@@ -72,3 +72,9 @@
 	  See `doc/arch/sandbox.rst` for more information.
 
 endmenu
+
+config FWU_NUM_BANKS
+	default 2
+
+config FWU_NUM_IMAGES_PER_BANK
+	default 2