powerpc: unify two CONFIG_POWERPC64_CPU entries in the same choice block

There are two CONFIG_POWERPC64_CPU entries in the "CPU selection"
choice block.

I guess the intent is to display a different prompt depending on
CPU_LITTLE_ENDIAN: "Generic (POWER5 and PowerPC 970 and above)" for big
endian, and "Generic (POWER8 and above)" for little endian.

I stumbled on this tricky use case, and worked around it on Kconfig with
commit 4d46b5b6 ("kconfig: fix infinite loop in sym_calc_choice()").
However, I doubt that supporting multiple entries with the same symbol
in a choice block is worth the complexity - this is the only such case
in the kernel tree.

This commit merges the two entries. Once this cleanup is accepted in
the powerpc subsystem, I will proceed to refactor the Kconfig parser.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250607102005.1965409-1-masahiroy@kernel.org