ARM: imx: Select GENERIC_ALLOCATOR

Since commit 657eee7d (media: coda: use genalloc API) the following build
error happens with imx_v4_v5_defconfig:

drivers/built-in.o: In function 'coda_remove':
clk-composite.c:(.text+0x112180): undefined reference to 'gen_pool_free'
drivers/built-in.o: In function 'coda_probe':
clk-composite.c:(.text+0x112310): undefined reference to 'of_get_named_gen_pool'
clk-composite.c:(.text+0x1123f4): undefined reference to 'gen_pool_alloc'
clk-composite.c:(.text+0x11240c): undefined reference to 'gen_pool_virt_to_phys'
clk-composite.c:(.text+0x112458): undefined reference to 'dev_get_gen_pool'

Select GENERIC_ALLOCATOR and get rid of the custom IRAM_ALLOC.

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