[CORE] Introduce vmm_openconf.h for config option checking

Currently, we did not have common config option checking
macros like linux/kconfig.h.

This patch adds vmm_openconf.h which will be always included
for all source files and it will provide config option
checking macros such as IS_ENABLED(x).

Signed-off-by: Anup Patel <anup@brainfault.org>
[DRIVERS] Add the PWM API and i.MX PWM controller support.

This patch add the support for PWM devices and the i.MX PWM controller driver
using it, with the PWM 4 device tree node, used by the LVDS backlight.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>
[DRIVERS] Improving i.MX UART driver for clock management.

The i.MX UART clock names are now scanned from the device tree, enabled, and
the uart_clock is set to the rate also precised in the device tree.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>