ARM: mm: update CONTEXTIDR register to contain PID of current process

This patch introduces a new Kconfig option which, when enabled, causes
the kernel to write the PID of the current task into the PROCID field
of the CONTEXTIDR on context switch. This is useful when analysing
hardware trace, since writes to this register can be configured to emit
an event into the trace stream.

The thread notifier for writing the PID is deliberately kept separate
from the ASID code, so that we can easily support newer processors (A15
onwards) which store the ASID in TTBR0. As such, the switch_mm code is
updated to perform a read-modify-write sequence to ensure that we don't
clobber the PID on older CPUs.

Change-Id: I7236834cf4b5e984c9d9f24ba6b872078c2b936f
Cc: Wolfgang Betz <wolfgang.betz@st.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>