[MIPS] EMMA2RH: Disable GEN_RTC, it can't possibly work.

Neither rtc_mips_get_time nor rtc_mips_set_time are being initialized by
the EMMA2RH setup code, so genrtc at best was a RTC dummy avoiding a few
error messages but not providing actual functionality.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>