rpmsg: glink: spi: Write unaligned data with short pkt

Writes on the SPI bus must be aligned to 4 bytes. If the data is not 4
byte aligned, first send an aligned chunk and then the rest of the data
as part of a short data pkt.

Change-Id: Ib9eea28d6c6e598aa7f7981bdb237c1305cce0d3
Signed-off-by: Chris Lew <clew@codeaurora.org>