m68k/atari: Hide RTC_PORT() macro from rtc-cmos

Rename RTC_PORT() to ATARI_RTC_PORT(), as the rtc-cmos RTC driver uses the
presence of this macro to enable support for the second NVRAM bank, which
Atari doesn't have ("Unable to handle kernel access at virtual address
00ff8965").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>