RTX5: uVisor: Defer to uVisor for SVCall priority
RTX5: uVisor: Defer to uVisor for SVCall priority

Only set the SVCall priority if uVisor is not present. If uVisor is
present, keep using whatever priorities it has already set up.

c1728p9/mbed-os,c1728p9/mbed-os,betzw/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,betzw/mbed-os,mbedmicro/mbed,andcor02/mbed-os,betzw/mbed-os,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os