lib: sbi: Simplify console platform operations

Instead of having console_putc() and console_getc() callbacks in
platform operations, it will be much simpler for console driver to
directly register these operations as device to the sbi_console
implementation.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>