package/freescale-imx/firmware-imx: custom padding

Some derivatives (such as Variscite imx8mm) expect the (LP)DDR4
training data to be padded to a different length.

eg: https://github.com/varigit/uboot-imx/commit/5f8d814f



This patch makes the padding length configurable.

Signed-off-by: Tibault Damman <tibault.damman@basalte.be>
Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>
Tested-by: Stephane Viau <stephane.viau@oss.nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>