mtd: m25p80: Make fast read configurable via DT

Add DT property "m25p,fast-read" that signalises the particular
chip supports "fast read" opcode.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>