lib: sbi: Detect Zicntr extension only based on traps

OpenSBI uses time CSR if Zicntr extension present which causes
it to crash on an older QEMU because QEMU generates Zicntr in
the ISA string for unleashed machine which only has CYCLE and
INSTRET counters.

Fixes: 776770d2

 ("lib: sbi: Using one array to define the
name of extensions")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>