[MIPS] SMTC: Allow control over TC assignment to vpe0.

Modify the SMTC initialization code to allow boot-time specification not
only of how many VPEs and TCs to use, but also how many TCs out of the
allowed pool are to be bound to VPE 0.  The new boot option is "vpe0tcs=N",
where N is an integer.  Using it in combination with the existing options
allows arbitrary assignments across the 2 VPEs of a 34K.  e.g. "maxtcs=3
 vpe0tcs=1" forces VPE0 to have 1 TC, while VPE1 has 2, and "maxtcs=4
vpe0tcs=3" forces VPE0 to have 3 TCs, while VPE1 gets 1.  If no vpe0tcs
option is specified, the traditional algorithm of evenly dividing TCs
between available VPEs, with the odd "slop" going to VPE0, is retained.

The reason for doing this is to allow a finer balancing of TCs which can
handle I/O interrupts on Malta (those on VPE 0) and those which cannot.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>