dmaengine: mmp_tdma: add missing select GENERIC_ALLOCATOR in Kconfig

There are some compilation errors when CONFIG_MMP_TDMA is enabled and
CONFIG_GENERIC_ALLOCATOR is disabled:

drivers/built-in.o: In function `mmp_tdma_prep_dma_cyclic':
mmp_tdma.c:(.text+0x7890e): undefined reference to `gen_pool_dma_alloc'
drivers/built-in.o: In function `mmp_tdma_free_chan_resources':
mmp_tdma.c:(.text+0x78aca): undefined reference to `gen_pool_free'
drivers/built-in.o: In function `mmp_tdma_probe':
mmp_tdma.c:(.text+0x78ea8): undefined reference to `of_gen_pool_get'

This commit fix this problem by selecting GENERIC_ALLOCATOR when
CONFIG_MMP_TDMA is enabled.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>