openrisc: Fix compiler warnings in setup

openrisc: Fix compiler warnings in setup

This was pointed out with the recent name change of or32_early_setup to
or1k_early_setup.  Investigating the file I found a few other warnings
so cleaning them up here.

    arch/openrisc/kernel/setup.c:220:13: warning: no previous prototype for 'or1k_early_setup' [-Wmissing-prototypes]
      220 | void __init or1k_early_setup(void *fdt)
	  |             ^~~~~~~~~~~~~~~~

Fix this the missing or1k_early_setup prototype warning by adding an
asm/setup.h file to define the prototype.

    arch/openrisc/kernel/setup.c:246:13: warning: no previous prototype for 'detect_unit_config' [-Wmissing-prototypes]
      246 | void __init detect_unit_config(unsigned long upr, unsigned long mask,
	  |             ^~~~~~~~~~~~~~~~~~

The function detect_unit_config is not used, just remove it.

    arch/openrisc/kernel/setup.c:221: warning: Function parameter or member 'fdt' not described in 'or1k_early_setup'

Add @fdt docs to the function comment to suppress this warning.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>