drivers: clock: Microchip XEC: Fix enum usage

We get a compiler warning in this code with arm clang due to using
the wrong enum type for the variable.  The enum should be of
type `enum periph_clk32k_src` so replace VBR_CLK32K_SRC_PIN_XTAL
with PERIPH_CLK32K_SRC_PIN_XTAL.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>