Move integer literal processing back to the lexer

At the moment integer literals are passed from the lexer to the parser as
a string, where it's evaluated into an integer by eval_literal().  That
strange approach happened because we needed to know whether we were
processing dts-v0 or dts-v1 - only known at the parser level - to know
how to interpret the literal properly.

dts-v0 support has been gone for some time now, and the base and bits
parameters to eval_literal() are essentially useless.

So, clean things up by moving the literal interpretation back to the lexer.
This also introduces a new lexical_error() function to report malformed
literals and set the treesource_error flag so that they'll cause a parse
failure at the top level.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>