arm64: defconfig: Enable additional Broadcom STB drivers

Enable the following drivers as modular:

- NAND controller with CONFIG_MTD_NAND_BRCMNAND
- AHCI/SATA controller driver with CONFIG_AHCI_BRCM
- Starfighter 2 Ethernet switch driver with CONFIG_NET_DSA_BCM_SF2
- USB drivers (OHCI, EHCI, XHCI) with CONFIG_USB_BRCMSTB
- Watchdog with CONFIG_BCM7038_WDT
- PWM with CONFIG_PWM_BRCMSTB
- SYSTEMPORT Ethernet controller with CONFIG_SYSTEMPORT

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>