env: use cache line aligned memory for flash read

Use memalign() with ARCH_DMA_MINALIGN to allocate read buffers.
This is required because, flash drivers may use DMA for read operations
and may have to invalidate the buffer before read.

Signed-off-by: Ravi Babu <ravibabu@ti.com>
Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>
Tested-by: Jagan Teki <jteki@openedev.com>