spi: ti_qspi: Use 4-byte opcode for mmap read

ti-qspi driver currently uses 3-byte addressing mode(and opcodes) for
memory-mapped read. This restricts maximum addressable flash size to
16MB.
Enable the 4-byte addressing(and use 4-byte opcode) for memory-mapped
read to allow access to addresses above 16MB.

Signed-off-by: Ravi Babu <ravibabu@ti.com>
[vigneshr@ti.com: Re-word commit description]
Signed-off-by: Vignesh R <vigneshr@ti.com>