memory: ti-gpmc: Fix lock up at A53 SPL during NAND boot on AM64-EVM

AM64 ES2.0 bootrom seems to enable WAIT0EDGEDETECTION interrupt.
This causes a lockup at A53 SPL when accessing NAND controller
or ELM registers.

A good option would be to softrest GPMC block at probe
but this cannot be done for AM64 as SOFTRESET bit is marked
as reserved in SYSCONFIG register.

Fix the issue by disabling all IRQs at probe.

Signed-off-by: Roger Quadros <rogerq@kernel.org>