ARM: ARMv7-M: implement read_cpuid_ext

On v7-M the extended cpuid registers are not available from CP15 but they
are memory mapped in the System Control Space.
There isn't an equivalent available for CPUID_{CACHETYPE,TCM,TLBTYPE,MPIDR}.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>