RISC-V: Add early printk support via the SBI console

This code lives entirely within the RISC-V arch code.  I've left it
within an "#ifdef CONFIG_EARLY_PRINTK" despite always having
EARLY_PRINTK support on RISC-V just in case someone wants to remove
it.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>