m68k: atari: Rename shifter to shifter_st to avoid conflict

When test-compiling the BCM2835 pin control driver on m68k:

    In file included from arch/m68k/include/asm/io_mm.h:32:0,
                     from arch/m68k/include/asm/io.h:8,
                     from include/linux/io.h:13,
                     from include/linux/irq.h:20,
                     from include/linux/gpio/driver.h:7,
                     from drivers/pinctrl/bcm/pinctrl-bcm2835.c:17:
    drivers/pinctrl/bcm/pinctrl-bcm2835.c: In function 'bcm2711_pull_config_set':
    arch/m68k/include/asm/atarihw.h:190:22: error: expected identifier or '(' before 'volatile'
     # define shifter ((*(volatile struct SHIFTER *)SHF_BAS))

"shifter" is a too generic name for a global definition.

As the corresponding definition for Atari TT is already called
"shifter_tt", fix this by renaming the definition for Atari ST to
"shifter_st".

Reported-by: kbuild test robot <lkp@intel.com>
Suggested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>