mtd: msm_qpic_nand: Changes to read boot config register

The resources (such as clocks) needed to enable/probe for NAND
are not available on eMMC based builds. Hence, enabling NAND
will cause boot-up failure on eMMC based builds. To avoid this,
fail NAND driver probe if the boot device used is not NAND.

Change-Id: I9fd83511efb6d413c5cd9f64357a8554f585aa6b
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>