Make SingletonPtr methods const
Make SingletonPtr methods const

Make get() and operators * and -> of SingletonPtr const - they are
logically const and thread-safe, despite the construction on first call.
This construction is "invisible" to the caller of those methods.

andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed