ARM: tegra: add pll_x freq table entry for 750MHz

Some SKUs limit the maximum CPU frequency to 750MHz; see
tegra2_pllx_clk_init(). The pll_x frequency table needs an entry for this
frequency, or there will be continual log spam from the cpufreq driver
attempting to set this rate, yet there being no table entry for it.

Signed-off-by: Stephen Warren <swarren@nvidia.com>