platform: Use CSR_<FOO> instead of <foo> for csr_*()

Some older toolchains may not have all the csr's defined. Update all
the csr functions to use the CSR_ #define values instead of the
toolchain defined values.

Suggested-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Atish Patra <atish.patra@wdc.com>