include: sbi: Few cosmetic changes in riscv_encoding.h

This patch does following cosmentic changes to riscv_encoding.h:
1. Re-organize CSR defines to match CSR listing in latest RISC-V
   privilege spec
2. Rename CSR_HCOUNTERNEN to CSR_HCOUNTEREN

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>