arm64: dts: renesas: Deduplicate extalr_clk bootph-all

arm64: dts: renesas: Deduplicate extalr_clk bootph-all

Most R-Car Gen3 and Gen4 systems require extalr_clk very early
in the boot process. Currently the extalr_clk { bootph-all } DT
property to indicate this clock should be available early is set
in each SoC U-Boot DT extras file. Deduplicate this assignment to
new common r8a779x-rcar64-u-boot.dtsi file.

Note that R-Car Gen3 R8A77990 E3 and R8A77995 D3 and RZ R8A774C0
do not require extalr_clk early, therefore these SoCs still include
plain r8a779x-u-boot.dtsi in their U-Boot DT extras .

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>