lib: sbi_tlb: Reduce size of struct sbi_tlb_info

Let us reduce the size of struct sbi_tlb_info by doing the
following:
1) Change the data type of asid and vmid fields to uint16_t
2) Replace local_fn() function pointer with an enum

Based on the above, the size of struct sbi_tlb_info is reduced
by 16 bytes on RV64 and 4 bytes on RV32.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>