lib: sbi: change sbi_hart_features.extensions as an array

In the future there may be a lot of ISA extensions, a 'long' may not
be able to accommodate, changed to an array for the future.

Addresses-Coverity-ID: 1568357 Out-of-bounds access
Fixes: 6259b2ec

 ("lib: utils/fdt: Fix fdt_parse_isa_extensions()
implementation")
Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Anup Patel <anup@brainfault.org>