ARM: OMAP: Fix gpio.c compile on 15xx with CONFIG_DEBUGFS

There are no wakeup registers on 15xx, and suspend_wakeup
does not exist in the struct gpio_bank.

Without this fix we'll get "arch/arm/plat-omap/gpio.c:1792:
error: 'struct gpio_bank' has no member named 'suspend_wakeup'"
as noted by Russell King.

Note that the ifdefs will be cleaned up once the omap gpio
code gets split into omap1 and omap2 specific parts.

Signed-off-by: Tony Lindgren <tony@atomide.com>