core: riscv: plat-spike: console driver based on host-target interface

Spike doesn't yet model a UART but relies on RISC-V Host-Target Interface
(HTIF) to perform all I/O. It is a protocol allowing

core: riscv: plat-spike: console driver based on host-target interface

Spike doesn't yet model a UART but relies on RISC-V Host-Target Interface
(HTIF) to perform all I/O. It is a protocol allowing the target to access
host to perform console, storage etc. It requires special ELF symbols
tohost and fromhost. HTIF base address is set to 0x40008000.

Signed-off-by: Marouene Boubakri <marouene.boubakri@nxp.com>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
[jf: remove useless line continuation; initialize base to 0]
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>

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