dm: core: support reading a single indexed u64 value

Add helper function to allow reading a single indexed u64 value from a
device-tree property containing multiple u64 values, that is an array of
u64's.

Co-developed-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/08043c8d204d0068f04c27de86afe78c75c50b69.1692956263.git.michal.simek@amd.com
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 63fda15..d4693e3 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -301,6 +301,7 @@
 		uint-value = <(-1234)>;
 		int64-value = /bits/ 64 <0x1111222233334444>;
 		int-array = <5678 9123 4567>;
+		int64-array = /bits/ 64 <0x1111222233334444 0x4444333322221111>;
 		str-value = "test string";
 		interrupts-extended = <&irq 3 0>;
 		acpi,name = "GHIJ";