MPCore: Fix missing header file inclusion; Add GIC-based implementations of up_enabable_irq(0 and up_disable_irq()

Rename current_regs to g_current_regs; For ARM, g_current_regs needs to be an array to support multiple CPUs

i.MX6: Fill in some 'Missing logic' that depended on CCM definitions.  Correct confusion with boot media configuration.

i.MX6 UART: Update periperal clock logic; Remove use of UART bits from i.MX1 that don't exist in i.MX6