riscv: defconfig: enable gpio support for HiFive Unleashed

Ethernet phy VSC8541-01 on HiFive Unleashed has its reset line
connected to a gpio, so enable GPIO driver's required to reset
the phy.

Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>