riscv: add prototypes for assembly language functions from head.S

Add prototypes for assembly language functions defined in head.S,
and include these prototypes into C source files that call those
functions.

This patch resolves the following warnings from sparse:

arch/riscv/kernel/setup.c:39:10: warning: symbol 'hart_lottery' was not declared. Should it be static?
arch/riscv/kernel/setup.c:42:13: warning: symbol 'parse_dtb' was not declared. Should it be static?
arch/riscv/kernel/smpboot.c:33:6: warning: symbol '__cpu_up_stack_pointer' was not declared. Should it be static?
arch/riscv/kernel/smpboot.c:34:6: warning: symbol '__cpu_up_task_pointer' was not declared. Should it be static?
arch/riscv/mm/fault.c:25:17: warning: symbol 'do_page_fault' was not declared. Should it be static?

This change should have no functional impact.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>