ARM: mach-at91: armv7: fix multiple cpu_reset definition when enabling SYSRESET
When SYSRESET is enabled, cpu_reset function is also defined in
sysreset-uclass.c which lead to multiple definitions of this function
since reset.c is build unconditionally. Add a check in Makefile to build
this file only if SYSRESET isn't enabled.
SYSRESET can be enabled when building SYSRESET_PSCI for instance on this
platform.