soc: esp32s2: Drop IRAM_ATTR from forward declarations

This commit drops the `IRAM_ATTR` macro from the function declarations
because:

1. `IRAM_ATTR` macro makes use of the `__COUNTER__` preprocessor macro,
which increments for every macro invocation and causes the section
specified in the forward declaration to not match that of the
function definition.

2. Section attributes need not be specified for forward declarations.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>