dtoc: Support ACPI paths in of-platdata

The start of an ACPI path typically has backslashes in it. These are not
preserved during the translation from device tree to C code, since dtc
(correctly) uses the first backslash as an escape character, and dtoc
therefore leaves it out of the C string.

Fix this with special-case handling.

Signed-off-by: Simon Glass <sjg@chromium.org>