ARM: pxa: fix compilation problem on AM300EPD board

This board fails compilation like this:
arch/arm/mach-pxa/am300epd.c: In function ‘am300_cleanup’:
arch/arm/mach-pxa/am300epd.c:179:2: error: implicit declaration
of function ‘PXA_GPIO_TO_IRQ’ [-Werror=implicit-function-declaration]
  free_irq(PXA_GPIO_TO_IRQ(RDY_GPIO_PIN), par);

This was caused by commit 88f718e3
"ARM: pxa: delete the custom GPIO header"

This is because it was previously getting the macro PXA_GPIO_TO_IRQ
implicitly from <linux/gpio.h> which in turn implicitly included
<mach/gpio.h> which in turn included <mach/irqs.h>.

Add the missing include so that the board compiles again.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>