This commit removes the inclusion of kernel_includes.h from
the nRF51 and nRF52 soc.h headers. This prevents from an
inclusion cycle formation on soc.h. In the wake of
kernel_includes.h removal, necessary header files have been
added in several source files to be able to compile Zephyr for
nRF5x SOCs.
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>