[PowerPC] Fix inclusion of dfp pseudoregs in tdep

[PowerPC] Fix inclusion of dfp pseudoregs in tdep

Previously, decimal floating point pseudoregisters were always included
in the target if it had a floating point unit.

This patch changes this to only include them if the target description
indicates that they are present, i.e. if the FPSCR register has more
than 32 bits.

gdb/ChangeLog:
2018-05-22  Pedro Franco de Carvalho  <pedromfc@linux.vnet.ibm.com>

	* rs6000-tdep.c (rs6000_gdbarch_init): Assign 1 to have_dfp only
	if the size of fpscr is larger than 32 bits.