spi: fsl-espi: pre-allocate message buffer

Currently the driver allocates a 64kb buffer for each single message.
On systems with little and fragmented memory this can result in
memory allocation errors. Solve this by pre-allocating a buffer.

This patch was developed in OpenWRT long ago, however it never
made it upstream.

I slightly modified the original patch to re-initialize the buffer
at the beginning of each transfer.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>