[ARM] Fix warning in arch/arm/mach-pxa/generic.c

Fix
 arch/arm/mach-pxa/generic.c:242: warning: 'struct i2c_pxa_platform_data' declared inside parameter list
caused by missing asm/arch/i2c.h include.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>