lib: sbi: Update the name of ISA string printed at boot time

The ISA string printed at boot time is not the complete ISA string
representing all single letter and multi-letter extensions rather
it is base ISA string derived from misa CSR so let us update the
boot print accordingly.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>