firmware: fw_base: Optimize _hartid_to_scratch() implementation

This patch optimizes _hartid_to_scratch() in following ways:
1. Use caller saved registers instead of callee saved registers
   so that we don't need to save/restore registers on stack
2. Remove second redundant mul instruction by re-arranging
   instructions

Overall, we reduce 9 instructions in _hartid_to_scratch()
implementation.

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