target/arm: add data cache invalidation cp15 instruction to cortex-r5

The cp15, CRn=15, opc1=0, CRm=5, opc2=0 instruction invalidates all the
data cache on the cortex-r5. Implementing it as a NOP.

Signed-off-by: Luc MICHEL <luc.michel@git.antfield.fr>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
