[ARM] Assign cost of scaling for Cortex-R52

This patch assigns cost of the scaling used in addressing for Cortex-R52.

On Cortex-R52 a negated register offset takes longer than a non-negated
register offset, in a register-offset addressing mode.

Differential Revision: http://reviews.llvm.org/D25670

Reviewer: jmolloy
llvm-svn: 284460