RISC-V: Avoid corrupting the upper 32-bit of phys_addr_t in ioremap

For 32bit, the upper 32-bit of phys_addr_t will be flushed to zero
after AND with PAGE_MASK because the data type of PAGE_MASK is
unsigned long. To fix this problem, the page alignment is done by
subtracting the page offset instead of AND with PAGE_MASK.

Signed-off-by: Vincent Chen <vincentc@andestech.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>