drivers/mtd/nvmxip: Rework the read accessor to support 32bit systems

Get rid of nvmxip_mmio_rawread() and just implement the readl()/readq()
reader loop within nvmxip_blk_read(). Cast the destination buffer as
needed and increment the read by either 4 or 8 bytes depending on if
this is systemd with 32bit or 64bit physical address.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>