ARM: 7164/3: PL330: Fix the size of the dst_cache_ctrl field

dst_cache_ctrl affects bits 3, 1 and 0 of AWCACHE but it is a 3-bit
field in the Channel Control Register (see Table 3-21 of the DMA-330
Technical Reference Manual) and should be programmed as such.

Reference: <1320244259-10496-3-git-send-email-javi.merino@arm.com>

Signed-off-by: Javi Merino <javi.merino@arm.com>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>