nitrogen6_max: eMMC operates at 1.8V

before:
        root@nitrogen:~# cat /sys/kernel/debug/mmc2/ios
        clock:		52000000 Hz
        actual clock:	49500000 Hz
        vdd:		17 (2.9 ~ 3.0 V)
        bus mode:	2 (push-pull)
        chip select:	0 (don't care)
        power mode:	2 (on)
        bus width:	3 (8 bits)
        timing spec:	7 (sd uhs DDR50)
        signal voltage:	0 (3.30 V)

after:
        root@nitrogen:~# cat /sys/kernel/debug/mmc2/ios
        clock:		52000000 Hz
        actual clock:	49500000 Hz
        vdd:		7 (1.65 - 1.95 V)
        bus mode:	2 (push-pull)
        chip select:	0 (don't care)
        power mode:	2 (on)
        bus width:	3 (8 bits)
        timing spec:	7 (sd uhs DDR50)
        signal voltage:	0 (1.80 V)

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>