dm: core: support reading a single indexed u64 value

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