dtc: Support character literals in cell lists

With this patch the following property assignment:

    property = <0x12345678 'a' '\r' 100>;

is equivalent to:

    property = <0x12345678 0x00000061 0x0000000D 0x00000064>

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>