[DRIVES] usb: Simplify root hub polling

We treat root hub in the same manner as normal hubs and expect
HCD driver to provide complete emulation of root hub.

Signed-off-by: Anup Patel <anup@brainfault.org>
[DRIVERS] Make clkdev.h xvisor specific

We have ported version of clk/clkdev.c hence the interface header
has to be xvisor specific drv/clkdev.h

Also, for linux compatibility we have linux/clkdev.h which simply
includes drv/clkdev.h

Signed-off-by: Anup Patel <anup@brainfault.org>
[DRIVERS] clk: Explicitly export symbols

Just like MMC and other driver frameworks explicitly export
symbols for lookup clock APIs even if it is always built-in.

Signed-off-by: Anup Patel <anup@brainfault.org>
[LIBS] libfdt: Don't access attr->value directly

The FDT parsing code directly accesses 'struct vmm_devtree_attr' member
'value' directly when parsing node properties. This not consistent with
other parts of Xvisor hence this patch updates FDT parsing accordingly.

Signed-off-by: Anup Patel <anup@brainfault.org>