armv8: cache_v8: fix mmu_set_region_dcache_behaviour

The enum dcache_optoion contains a shift left 2 bits in the armv8 case
already.  The PMD_ATTRINDX(option) macro will perform a left shift of 2
bits.  Perform a right shift so that in the end we get the correct
value.

[trini: Reword the commit message]
Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>