tqm834x: Expand CONFIG_SYS_OR_TIMING_FLASH macro

We want to normalize all BR/OR config lines as much as possible.

The TQM834x board uses CONFIG_SYS_OR_TIMING_FLASH in a OR definition,
which we want to remove. But CONFIG_SYS_OR_TIMING_FLASH is also used
outside of the config file.

Replace these usages with the definition of the variable, so we can
remove the variable in the next patch.

Signed-off-by: Mario Six <mario.six@gdsys.cc>