Update README_Xperia

Update to correct configs found in arch/arm/configs and cross compiler information based on guide: http://developer.sonymobile.com/knowledge-base/tutorials/linux-rom/how-to-rebuild-kernels-for-flagship-xperia-devices/

Add ARCH=arm CROSS_COMPILE=$CROSS_COMPILE to line 39 for correct configuration
Merge pull request #38 from necioerrante/patch-1

Update README_Xperia