arch/powerpc: get rid of BR2_GCC_TARGET_TUNE

According to the gcc documentation for PowerPC options:

'-mtune=CPU_TYPE'
     Set the instruction scheduling parameters for machine type
     CPU_TYPE, but do not set the architecture type, register usage, or
     choice of mnemonics, as '-mcpu=CPU_TYPE' would.  The same values
     for CPU_TYPE are used for '-mtune' as for '-mcpu'.  If both are
     specified, the code generated will use the architecture, registers,
     and mnemonics set by '-mcpu', but the scheduling parameters set by
     '-mtune'.

In the case of Buildroot where we only target a specific system, using
-mtune therefore doesn't make much sense, and using -mcpu would be
more appropriate. As a consequence, this patch makes PowerPC use
BR2_GCC_TARGET_CPU instead of BR2_GCC_TARGET_TUNE.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>