Fix: remove regmap instrumentation for kernels < 4.1

Modifications to regmap.h instrumentation in mainline Linux kernel has
been no less than erratic. First, the public instrumentation header
regmap.h include a private driver header in Linux 4.0. Then, regmap.h
is moved to the private driver directory in Linux 4.1. To make things
worse, the 4.0 commit has been picked into stable branches of the Linux
kernel.

Since this does not appear to be an instrumentation of utmost
importance, only build this instrumentation probe if the private header
is found in the driver directory (need full kernel sources).

This removes regmap instrumentation for older kernels.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>