lib: sbi: Add Zicntr as a HART ISA extension

Recently ratified Zicntr ISA extension covers cycle, time and
instret CSRs so we replace the "time" ISA extension with "zicntr"
ISA extension in OpenSBI.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>