This patch is to support mx6ul_9x9_evk board based on mx6ul_14x14_evk,
the difference between mx6ul 9x9 evk and mx6ul 14x14 evk are:
1. mx6ul 9x9 evk use pfuze3000, while mx6ul 14x14 evk use DCDC.
2. mx6ul 9x9 evk supports 256MB LPDDR2, while mx6ul 14x14 evk
   supports 512MB DDR3
3. mx6ul_9x9_evk use 9x9 package, while mx6ul_14x14_evk use 14x14 package.

This patch add the following:
1. Discard PHYS_SDRAM_SIZE from header file, use imx_ddr_size()
2. Introduce a macro is_mx6ul_9x9_evk using
   CONFIG_IS_ENABLED(TARGET_MX6UL_9X9_EVK) to avoid "#ifdef xxx" in non-SPL
   part. To SPL part, CONFIG_IS_ENABLED(TARGET_MX6UL_9X9_EVK) can not work,
   so still use "#ifdef CONFIG_TARGET_MX6UL_9X9_EVK" to differentiate with
   mx6ul_14x14_evk. And we have no way to dymaically checking this chip
   is 9x9 or 14x14.
3. mx6ul_9x9_evk use pfuze3000, so enabled POWER related configurations.
   POWER related configurations also effect for mx6ul_14x14_evk. But
   power_init_board implementation using 'if (is_mx6ul_9x9_evk())' to
   do initialization for mx6ul_9x9_evk, and do nothing for mx6ul_14x14_evk.
4. mx6ul_9x9_evk use lpddr2 with size 256MB, so add related SPL DRAM
   configurations.
5. Enable CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG and setting dtb file
   according to board_rev and board_name.
6. Add TARGET_MX6UL_9X9_EVK Kconfig entry

Boot Log:
U-Boot SPL 2015.10-rc2-00356-g536ce34 (Sep 06 2015 - 12:22:53)
reading u-boot.img
reading u-boot.img

U-Boot 2015.10-rc2-00356-g536ce34 (Sep 06 2015 - 12:22:53 +0800)

CPU:   Freescale i.MX6UL rev1.0 792 MHz (running at 396 MHz)
CPU:   Commercial temperature grade (0C to 95C) at 41C
Reset cause: POR
Board: MX6UL 9x9 EVK
I2C:   ready
DRAM:  256 MiB
PMIC: PFUZE3000 DEV_ID=0x30 REV_ID=0x11
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
In:    serial
Out:   serial
Err:   serial
Net:   FEC1
Hit any key to stop autoboot:  0

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>