rt2800: 5592: early defines

Add basic defines for 5592 chip. It can not be enabled until
CONFIG_RT2800USB_RT55XX configuration option will be provided in the
Kconfig.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>