riscv: renames shadow variables

In print_pmp_entries(), start and end are function arguments
and yet another start and end are declared inside the for
loop. So rename the function arguments to fix shadow variables
warning.

The changes in csr_*() macros are needed to avoid shadowing
__v when nesting those functions together, for example,
csr_write(..., csr_read(...)).

Signed-off-by: Daniel Leung <daniel.leung@intel.com>