arm: cache: Add support for write-allocate D-Cache

Add configuration for the write-allocate mode of L1 D-Cache on ARM.
This is needed for D-Cache operation on Cortex-A9 on the SoCFPGA .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Pavel Machek <pavel@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>