lib: sbi: pmu: fix usage of sbi_pmu_irq_bit()

While sbi_pmu_irq_bit() was used to delegate irq to S-mode, LCOFIP usage
was still hardcoded in various places. This led to change the returned
value of sbi_pmu_irq_bit() to be a bit number rather than a bit mask
since it returns an 'int' and we need to obtain the bit number itself to
handle it in the IRQs handlers. Add a similar function to return the
irq mask which can also be used where the mask is required rather than
the bit itself.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>

include/sbi/sbi_pmu.h

@@ -114,6 +114,9 @@ void sbi_pmu_exit(struct sbi_scratch *scratch);
 /** Return the pmu irq bit depending on extension existence */
 int sbi_pmu_irq_bit(void);
 
+/** Return the pmu irq mask or 0 if the pmu overflow irq is not supported */
+unsigned long sbi_pmu_irq_mask(void);
+
 /**
  * Add the hardware event to counter mapping information. This should be called
  * from the platform code to update the mapping table.