arm64: dts: imx8m: remove explicit spl filename

Binman currently ignores the specified filename and always uses
'spl/u-boot-spl.bin'. The dtsi files modified in this patch all specify
'u-boot-spl.bin' as the filename for u-boot-spl, which only exists in
the spl subdirectory. This patch removes the explicit filename so the
build does not break when Binman is fixed in the next patch in this
series.

Signed-off-by: Harm Berntsen <harm.berntsen@nedap.com>
CC: Marcel Ziswiler <marcel.ziswiler@toradex.com>
CC: Peng Fan <peng.fan@nxp.com>
CC: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
CC: Fabio Estevam <festevam@gmail.com>
CC: Tim Harvey <tharvey@gateworks.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Patrick Wildt <patrick@blueri.se>