hw/riscv: Replace global smp variables with machine smp properties

hw/riscv: Replace global smp variables with machine smp properties

The global smp variables in riscv are replaced with smp machine properties.

A local variable of the same name would be introduced in the declaration
phase if it's used widely in the context OR replace it on the spot if it's
only used once. No semantic changes.

Signed-off-by: Like Xu <[email protected]>
Message-Id: <20190518205428[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
[ehabkost: fix spike_board_init()]
[ehabkost: fix riscv_sifive_e_soc_init()]
Signed-off-by: Eduardo Habkost <[email protected]>