powerpc/signal: Fix confusing header documentation in sigcontext.h

Commit ce48b210 "powerpc: Add VSX context save/restore, ptrace and
signal support" expanded the 'vmx_reserve' array element to contain 101
double words, but the comment block above was not updated.

Also reorder the constants in the array size declaration to reflect the
logic mentioned in the comment block above. This change helps in
explaining how the HW registers are represented in the array. But no
functional change.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Reworded change log and added whitespace around +'s]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>