OMAP2: serial.c: Fix number of uarts in early_init

The omap_serial_early_init prints the following errors:

Could not get uart4_ick
Could not get uart4_fck

because all the uarts available in omap_uart[] will be initialized.
Only omap4430 and omap3630 have 4 uarts at the moment.
This patch reduces the number of uarts when cpu is not omap4430 or
omap3630.

Signed-off-by: Thomas Weber <weber@corscience.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>