rockchip: spi: make optimised receive-handler unaligned-safe

To support unaligned output buffers (i.e. 'in' in the terminology of
the SPI framework), this change splits each 16bit FIFO element after
reading and writes them to memory in two 8bit transactions.  With this
change, we can now always use the optimised mode for receive-only
transcations independent on the alignment of the target buffer.

Given that we'll run with caches on, the impact should be negligible:
as expected, this has no adverse impact on throughput if running with
a 960MHz LPLL configuration.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>