platform: Update UART base addresses for qemu/sifve_u

In the QEMU [1] there was a change of the UART base addresses for
sifive_u machine to match the hardware. Make corresponding changes in
the opensbi for qemu/sifive_u platform.

[1] https://git.qemu.org/?p=qemu.git;a=commitdiff;h=4b55bc2b5f7ff065da5d2b813ee5153c598d3764



Signed-off-by: Nikita Ermakov <coffe92@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>