RISC-V: fix virtual address overlapped in FIXADDR_START and VMEMMAP_START

This patch fixes the virtual address layout in pgtable.h.  The virtual
address of FIXADDR_START and VMEMMAP_START should not be overlapped.

Fixes: d95f1a54 ("RISC-V: Implement sparsemem")
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
[paul.walmsley@sifive.com: fixed patch description]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>