When parsing mti,reserved-cpu-vectors we generate a mask of all bits
that have been declared reserved, the loop through starting from bit 2
to find one that isn't reserved (ie. is zero).

This patch accomplishes the same task more simply by:

  - Inititialising the reserved mask to 0x3 (ie. the 2 software
    interrupts). This means we don't need to skip them later as the loop
    previously has.

  - Replacing the loop checking for zero bits with find_first_zero_bit,
    which fits our needs now that the 2 software interrupts are marked
    reserved. This requires that the type of reserved is changed to
    unsigned long so that it's suitable for use with bitmap functions.

  - Replacing the magic number 8 with the hamming weight of the ST0_IM
    field - ie. the number of bits that a MIPS CPU has for interrupt
    inputs. This is still a compile-time constant 8, but makes it
    clearer why it's 8.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17054/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>