ARM: tegra: pack pinmux data tables tighter

Use smaller fields in the Tegra pinmux structures in order to pack the
data tables into a smaller space. This saves around 1-3KB for the SPL
and around 3-8KB for the main build of U-Boot, depending on the board,
which SoC it uses, and how many pinmux table entries there are.

In order to pack PMUX_FUNC_* into a smaller space, don't hard-code the
values of PMUX_FUNC_RSVD* to values which require 16 bits to store them,
but instead let their values be assigned automatically, so they end up
fitting into 8 bits.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>