dtoc: Add a 64-bit type and a way to convert cells into 64 bits

When dealing with multi-cell values we need a type that can hold this
value. Add this and a function to process it from a list of cell values.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Kever Yang <kever.yang@rock-chips.com>