ARM: remove CONFIG_ARM920T defines

CONFIG_CPU_ARM920T was introduced into Kconfig by commit 2e07c249
(kconfig: arm: introduce symbol for ARM CPUs).

This commit removes all the defines of CONFIG_ARM920T and replaces the
only reference in drivers/usb/host/ohci-hcd.c with CONFIG_CPU_ARM920T.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>