arm: juno: Enable SATA controller

The ARM Juno boards (-r1 and -r2) feature a Silicon Image 3132 PCIe
SATA controller soldered on the board, providing two SATA ports.

Enable the driver and the sata command in the defconfig, to be able to
load images from SATA disks.

Tested by loading kernels and Grub/EFI from an SSD and successfully
booting a Linux system (with and without using UEFI).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>