cpu/rocket: access PLIC registers via pointer dereference

cpu/rocket: access PLIC registers via pointer dereference

Since the PLIC is internal to Rocket, access its registers
directly via pointer dereference, rather than through the
LiteX CSR Bus accessors (which assume subregister slicing,
and are therefore inappropriate for registers NOT accessed
over the LiteX CSR Bus).

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>