This patch adds SiFive FU540 board support. For now, only
SiFive serial, SiFive PRCI, and Cadance MACB drivers are
only enabled. The SiFive FU540 defconfig by default builds
U-Boot for S-Mode because U-Boot on SiFive FU540 will run
in S-Mode as payload of BBL or OpenSBI.

Signed-off-by: Atish Patra
Signed-off-by: Anup Patel
Reviewed-by: Alexander Graf
Reviewed-by: Lukas Auer
Reviewed-by: Bin Meng