libc: minimal: Align errno.h values with newlib

In order to be able to document the error codes with Doxygen and
Breathe, start by aligning the macro values to those provided by newlib
in include/sys/errno.h:

https://github.com/zephyrproject-rtos/newlib-cygwin/blob/zephyr-newlib-4.1.0/newlib/libc/include/sys/errno.h



Also get rid of the (unused and untrue) ERRMAX definition.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>