powerpc/52xx: Remove dead code, i.e. mpc52xx_get_xtal_freq()

It seems mpc52xx_get_xtal_freq() is not used anywhere. Remove dead code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Wolfram Sang <wsa@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220507100147.5802-1-andriy.shevchenko@linux.intel.com