riscv/vdso: Refactor asm/vdso.h

The asm/vdso.h will be included in vdso.lds.S in the next patch, the
following cleanup is needed to avoid syntax error:

 1.the declaration of sys_riscv_flush_icache() is moved into asm/syscall.h.
 2.the definition of struct vdso_data is moved into kernel/vdso.c.
 2.the definition of VDSO_SYMBOL is placed under "#ifndef __ASSEMBLY__".

Also remove the redundant linux/types.h include.

Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>