csr: use readl()/writel() accessors for accessing mmio

csr: use readl()/writel() accessors for accessing mmio

Instead of directly dereferencing pointers, use variants on readl()/writel().
This way we can replace these functions with others for remote access
when writing drivers and code outside of the litex environment.

Signed-off-by: Sean Cross <sean@xobs.io>