am335x: fix GPMC config for NAND and NOR SPL boot

GPMC controller is common IP to interface with both NAND and NOR flash devices.
Also, it supports max 8 chip-selects, which can be independently connected to
any of the devices.
But ROM code expects the boot-device to be connected to only chip-select[0].
Thus to resolve conflict between NOR and NAND boot. This patch:
- combines NOR and NAND configs spread in board files to common gpmc_init()
- configures GPMC based on boot-mode selected for SPL boot.

Signed-off-by: Pekon Gupta <pekon@ti.com>