include: Make sbi_current_hartid() as macro in riscv_asm.h

The sbi_current_hartid() being a regular function is quite
expensive because for callers it is a function call instead
of a direct CSR read. This patch converts sbi_current_hartid()
into a macro in riscv_asm.h.

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

include/sbi/sbi_hart.h

@@ -32,6 +32,4 @@ sbi_hart_switch_mode(unsigned long arg0, unsigned long arg1,
 		     unsigned long next_addr, unsigned long next_mode,
 		     bool next_virt);
 
-u32 sbi_current_hartid(void);
-
 #endif