arm: dts: bcm283x: Rename U-Boot file

Rename the file bcm283x-uboot.dtsi so that it get
automatically include through the scripts/Makefile.lib
using $(CONFIG_SYS_SOC))-u-boot.dtsi

Without this uarts and pincontroller miss the property dm-pre-reloc
and the first call to bcm283x_mu_serial_ofdata_to_platdata() fails
as the pins are not set correctly.
As a result the U-Boot banner isn't shown on boot.

Before commmit
143256b3 ("fdt: update bcm283x device tree sources to Linux 5.1-rc6 state")
we included bcm283x-uboot.dtsi directly in the device-tree file.
Which got deleted by the metioned commit.
This is a much robuster solution.

Reported-by: Tom Rini <trini@konsulko.com>
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Tom Rini <trini@konsulko.com> [RPi 3, 32b and 64b modes]
Signed-off-by: Matthias Brugger <mbrugger@suse.com>