ASoC: wcd-spi: add support to read requested section of memory

It is possible that the manager driver may try to read codec memory
section through the wcd-spi driver. Add support in the wcd-spi driver
to handle the WDSP_EVENT_READ_SECTION event and perform reading of
the requested memory section.

Change-Id: If7fb228dc15cb47079c8a791443c4d12c29eeb9c
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>