ARM: mach-imx: Select CONFIG_SRAM at ARCH_MXC level

ARM: mach-imx: Select CONFIG_SRAM at ARCH_MXC level

Booting a mx6q system built with multi_v7_defconfig leads to the following
error messages on boot:

[    0.037758] imx6q_ocram_suspend_init: ocram pool unavailable!
[    0.037768] imx6_pm_common_init: failed to initialize ocram suspend -19!

This happens because CONFIG_SRAM is not selected by default in
multi_v7_defconfig.

Fix this by selecting CONFIG_SRAM at ARCH_MXC level, so that other SoCs could
use the SRAM driver as well.

As SRAM automatically selects GENERIC_ALLOCATOR, just drop it from the Kconfig
entry.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>