ARM: mach-shmobile: clock-r8a7740: add USB clock

R8A7740 USB needs many clocks for workaround,
and it has confusing name "usb24s" and "usb24".
This "usb24s" will be used by other clocks.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Simon Horman <horms@verge.net.au>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>