ata: Kconfig: fix sata gemini compile test condition

When compile testing the sata gemini driver, CONFIG_OF is required to
avoid the warning:

drivers/ata/sata_gemini.c:421:34: error: ‘gemini_sata_of_match’ defined
but not used [-Werror=unused-const-variable=]

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>