powerpc/machdep: Remove stale functions from ppc_md structure

ppc_md.iommu_save() is not set anymore by any platform after
commit c40785ad ("powerpc/dart: Use a cachable DART").
So iommu_save() has become a nop and can be removed.

ppc_md.show_percpuinfo() is not set anymore by any platform after
commit 4350147a ("[PATCH] ppc64: SMU based macs cpufreq support").

Last users of ppc_md.rtc_read_val() and ppc_md.rtc_write_val() were
removed by commit 0f03a43b ("[POWERPC] Remove todc code from
ARCH=powerpc")

Last user of kgdb_map_scc() was removed by commit 17ce452f ("kgdb,
powerpc: arch specific powerpc kgdb support").

ppc.machine_kexec_prepare() has not been used since
commit 8ee3e0d6

 ("powerpc: Remove the main legacy iSerie platform
code"). This allows the removal of machine_kexec_prepare() and the
rename of default_machine_kexec_prepare() into machine_kexec_prepare()

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Daniel Axtens <dja@axtens.net>
[mpe: Drop prototype for default_machine_kexec_prepare() as noted by dja]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/24d4ca0ada683c9436a5f812a7aeb0a1362afa2b.1630398606.git.christophe.leroy@csgroup.eu