[PATCH] powerpc/ppc: Add missing isyncs in head_fsl_booke.S

The e500 core reference manual indicates that isync is required
after mtmsr(DE bit) and mtspr DBCR0.  Add isyncs to make the code
conform to the spec.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>