sh: Fix -Wmissing-include-dirs warnings for various platforms

sh: Fix -Wmissing-include-dirs warnings for various platforms

The 0day bot reports a lot of warnings (or errors due to CONFIG_WERROR)
like this:

  cc1: error: arch/sh/include/mach-hp6xx: No such file or directory [-Werror=missing-include-dirs]

Indeed, arch/sh/include/mach-hp6xx does not exist.

While -Wmissing-include-dirs is only a W=1 warning, it may be
annoying when CONFIG_BTRFS_FS is enabled because fs/btrfs/Makefile
unconditionally adds this warning option.

arch/sh/Makefile defines machdir-y for two purposes:

 - Build platform code in arch/sh/boards/mach-*/
 - Add arch/sh/include/mach-*/ to the header search path

For the latter, some platforms use arch/sh/include/mach-common/
instead of having its own arch/sh/include/mach-*/.

Drop unneeded machdir-y to omit non-existing include directories.

To build arch/sh/boards/mach-*/, use the standard obj-y syntax in
arch/sh/boards/Makefile.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202302190641.30VVXnPb-lkp@intel.com/
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230219141555.2308306-1-masahiroy@kernel.org
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>