clk: meson: fix MPLL 50M binding id typo

MPLL_5OM (the capital letter o) should indeed be MPLL_50M (the number)
Fix this before it gets used.

Fixes: 25db146a

 ("dt-bindings: clk: meson: add g12a periph clock controller bindings")
Reported-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>