include: sbi_platform: fix compilation for GCC-9

GCC-9 will throw a warning when using the %s format specifier with a
possible NULL parameter and since -Werror is used, the compilation breaks
for GCC-9.

In function 'sbi_boot_prints',
    inlined from 'init_coldboot' at <redacted>/opensbi/lib/sbi/sbi_init.c:107:3,
    inlined from 'sbi_init' at <redacted>/opensbi/lib/sbi/sbi_init.c:189:3:
<redacted>/opensbi/lib/sbi/sbi_init.c:56:2: error: '%s' directive argument is null [-Werror=format-overflow=]
   56 |  sbi_printf("Platform Name          : %s\n", sbi_platform_name(plat));
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

This is one way to fix this, currently there is nothing in the tree
checking for `sbi_platfrom_name() == NULL` so we can just return "Unknown"
instead of NULL on failure.

Signed-off-by: Martin Pietryka <martin@pietryka.at>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Tested-by: David Abdurachmanov <david.abdurachmanov@sifive.com>