msm: nand: Increase number of supported NAND partitions for FOTA

Bootloader now additionaly exposes modem partitions to mtd via atag to
support FOTA - delta update of modem images from recovery.

Number of partitions supported is increased to 18 to expose partitions
up until FOTA.

Change-Id: Icde54056403fe9b6a0a8f5c4554036cbe4d4abe0
Signed-off-by: Mekala Natarajan <mekalan@codeaurora.org>