powerpc/mpc52xx_gpt: make use of raw_spinlock variants

The mpc52xx_gpt code currently implements an irq_chip for handling
interrupts; due to how irq_chip handling is done, it's necessary for
the irq_chip methods to be invoked from hardirq context, even on a a
real-time kernel. Because the spinlock_t type becomes a "sleeping"
spinlock w/ RT kernels, it is not suitable to be used with irq_chips.

A quick audit of the operations under the lock reveal that they do
only minimal, bounded work, and are therefore safe to do under a raw
spinlock.

Signed-off-by: Julia Cartwright <julia@ni.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>